Dollos Law

7/25/2019 Dollos Law

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D o l l o o n D o l l o s L a w : I r r e v e r si b i li ty a n d

t h e S t a tu s o f E v o l u t io n a r y L a w s

S T E P H E N : lA Y G O U L D

M u s e u m o f C o m p a r a t iv e Z o o lo g y H a r v a r d U n i v e r s i t y

C a m b r i d g e M a s s a c h u s e t t s

I . D O L L O ' S F O R M U L A T I O N O F D O L L O ' S L A W

L r r ~ ve r s ib i li t6 : j e s u i s b i e n t r a n q u i l l e s u r l ' a v e n i r e t r u t i l i t 6 d e c e t t e

n o t i o n : s e u l e m e n t , p o u r l a s o u te r f i r o u p o u r l a c o m b a t t r e , i l f a u t b i e n l a

c o m p r e n d r e , c e q u i n ' a r r i v e p a s t o u j o u r s

L . D o l l o i n l e t t e r t o T . E d i n g e r , J u l y 9 , 1 9 2 7

O t h e n i o A b e l l a u n c h e d

alaeobiologica

w i t h a w i s h t h a t t h e

i d e a s o f L o u i s D o l lo m i g h t f l o u r i sh a n d b r i n g p r o s p e r i t y t o t h e

n e w j o u r n a l . 1 alaeobiologicad i d n o t s u r v i v e t h e w a r ; D o l l o ' s

n a m e l iv e s a s a m a s t h e a d t o t h e l a w o f i r r e v e rs i bf l i ty , b u t h i s

f o r g o t t e n w o r k p r e s e n t s t h i s n o t i o n i n a f a s h i o n a l t o g e t h e r

d i f f e re n t f r o m t h e f o r m u l a t i o n s o f o u r t e x t b o o k s. A b e l 's j o u r n a l

m a y h a v e m e t a k in d e r f a t e . D o l l o ' s l a w , m o r e o v e r , h a s f a l l e n

i n t o d i s r e p u t e a l o n g w i t h t h e e n t i r e e n t e r p r i s e t h a t s o u g h t to

a b s t r a c t h i s to r i c a l l a w s f r o m t h e p h e n o m e n a o f p h y l o g e n y . I

f in d th i s u n f o r t u n a t e f o r t w o r e a s o n s :

1. A p a r t f r o m a n y j u d g m e n t o n t h e m e r i t o f D o l l o 's l a w , I

r e g r e t th i s f o r e c l o s u r e o f d i s c u s s i o n s i n c e t h e d e b a t e o n h i s t o r ic a l

l a w s i l l u m i n a t e s s o m a n y i s su e s i n t h e p h i l o s o p h y o f b io l og y

( r e d u c t i o n i s m , t h e n a t u r e o f h i s t o r y ) .

2 . I r r e v e r s i b i l i t y , i n i ts m o s t i m p o r t a n t s e n s e , is a n o t i o n

q u i t e d i f f e re n t f r o m t h e s t a n d a r d s e t o f s u c h ' l a w s t h o s e

n a m e d f o r C o p e , W f l li s to n , e t c . B y a n i r o n i c t w i s t , a s w e s h a l l

s ee , D o l lo 's l a w e m e r g e s a s a p a r t i c u l a r i z e d s t a t e m e n t o f t h e

1. O t h e n i o A b e l , " D i e F e s t g a b e d e r " P a l a e o b i o l o g i ca , ' " Palaeob io log ica 1

( 1 9 2 8 ) , 1 - 8 .

J o u r n a l o f t h e H i s t o r y o f B i o l o g y v o l . 3 , n o . 2 ( F a l l 1 9 7 0 ) , p p . 1 8 9 - 2 1 2 .

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S T E P H E N J A Y G O U L D

g e n e r a l r e a s o n f o r o u r r e je c t i o n o f a n a p p r o a c h to p h y l o g e n y

b a s e d o n a s e a r c h f o r s u c h h i s t o r ic a l l a w s .

L o u is D o llo ( 1 8 5 7 - 1 9 3 1 ) w a s a m u c h m i s u n d e r s t o o d m a n .

B o r n a n d e d u c a t e d i n L i l l e , h e b e g a n a l i f e t i m e c a r e e r a t t h e

B r u s s e l s M u s e u m i n 1 889. a f t e r a b r i e f s t i n t a s a m i n i n g e n g i n e e r .

H i s o p e n s u p p o r t o f G e r m a n y d u r i n g t h e F i r s t W o r l d W a r p r e -

c l u d e d a n y f u t u r e p o p u l a r i t y w i t h h i s B e l g i a n c o l le a g u e s ; y e t

w i t h a s t u b b o r n n e s s t h a t h e a t t r ib u t e d t o h i s B r e t o n a n c e s t r y ,

h e r e m a i n e d a t h i s p o s t a s a v i r t u a l r e c lu s e . W h i le h e m a i n t a i n e d

a f e w l o y a l f r i e n d s a n d a u n i q u e l y h i g h s t a t u s i n t h e p a l e o n t o -

l o g ic a l c o m m u n i t y o f E u r o p e , h i s s e c lu s i o n a n d a v o i d a n c e o f

t h e p o l it i c s o f s c ie n t i f ic s o c i e t i e s f o s t e r e d h i s r e p u t a t i o n a s a

q u i e t l y d e d i c a t e d , c o l d ly d i s p a s s i o n a t e s c i e n ti s t . A ll t h a t h e h i d

f r o m h i s c o l l e a g u e s h e d i s c lo s e d i n t h e e x t r a o r d i n a r y c o r-

r e s p o n d e n c e w i t h D r . T f l ly E d i n g e r , f o u n d a f t e r t h e l a t t e r ' s

d e a t h i n 1 9 6 7; h e r e , i n t e r s p e r s e d w i th W a g n e r i a n q u o t a t io n s o f

d e a t h a n d y e a r n i n g , w e fi n d t h e w o r d s o f a l o n e ly a n d t o r m e n t e d

i d e a l i s t .

A s h e c o n c e a l e d h i s f e e l i n g s b y f o r c e o f p e r s o n a l i t y , s o al s o d id

h e w i t h h o l d h i s i d e a s b y h a b i t s o f w r i ti n g . H e w r o t e n e i t h e r t e x t

n o r r e v i e w a r t i c le , a n d w e p a l e o n t o l o g i s ts h a v e f o r g o t t e n t h a t h i s

p a ld o n to lo g ie d th o lo g iq u e w a s t h e s o u r c e f o r a t y p e o f r e s e a r c h

t h a t w e a l l p u r s u e t o d a y - - t h e s t u d y o f a d a p t a t i o n i n r e Ia t i o n s h i p

t o e n v i r o n m e n t . H e w r o t e n o d i s c u r s i v e p ro s e , n o e l a b o r a t i o n o f

g e n e r a l i d e as , b u t l i s t ed h i s c o n t e n t i o n s o n l y a s se t s o f s u m m a r y

p r o p o s i t i o n s . T h e q u o t e d s o u r c e f o r a l l h i s e v o l u t i o n a r y t h e o r iz -

i n g i s a t w o p a g e r 6 s u m ~ i n t he P r o c ~ s - V e b a u x o f 1 8 9 3 f o r t h e

g e o lo g i ca l s o c ie t y o f B e l g i u m ( t r a n s l a t e d a s a n a p p e n d i x to t h is

a r t i c l e ) ; r a r e l y d id h e e l a b o r a t e a n y f u r t h e r i n h i s l a t e r w o r k s .

H e w r o t e t o E d i n g e r o f h i s v i e w s o n s c ie n t i fi c p ro s e ( S e p t e m -

b e r 2 1, 1 9 2 9 ) : " D o U o 's s ty l e , a te l e g r a p h i c s t y l e - - d i f f i c u l t to

r e a d . . . Y e s , b u t

c l e a r , b r i e f , p r e c i s e - - t h a t

i s m y p u r p o s e

T h e c o n se q u e n c e o f a s t ro n g m a t h e m a t i c a l e d u c a t io n A n origi-

n a l

m e m o i r i s

n o t a s t o r y l

T h e s e e x a m p l e s o f h i s p u b l i s h e d

s t a t e m e n t s o n i r r e v e r s i b f l i t y a r e t y p i c a l :

A D o ll o e p i t o m e : 2

N a u t i l u s d o e s n o t y e t h a v e f i ns ; O c t o p u s h a s t h e m n o l o n g e r .

F r o m t h is p o i n t o f v i e w t h e s e r i e s e n d s w h e r e i t b e g i n s .

B u t t h e r e i s n o t h i n g c o n t r a r y t o i r r e v e r s i b i li t y i n th i s .

A f t e r a l l , O c t o p u s h a s n o t t u r n e d i n t o a n a u t f lo i d .

2 . 19 19 ., p . 1 1 7 . A c h r o n o l o g i c a l b i b l i o g r a p h y o D o l l o s w o r k o n I t -

r e v e r s ib i l i t y i s p r e s e n t e d a t t h e c l o s e o f t h i s a r t ic l e . C i t a t i o n s h e r e i n a r e b y

d a t e a n d r e f e r to t h e s e w o r k s .

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D o l lo o n D o l lo ' s L a w

T h e d id a c t i c D o l lo d e m o n s t r a t i n g t h a t u n r o l l e d a m m o n o i d s

h a v e n o t r e v e r t e d t o t h e a n c e s t r a l s t r a i g h t n a u t f l o i d :3

N o n e o f t h e m h a s b e c o m e t h e a n c e s t r a l Orthoceras a g a i n

. . . n e i t h e r w h o l l y , n o r p a r t i a l l y : n e i t h e r in t h e i n i t ia l

c h a m b e r , n o r in t h e s u t u r e s , n o r in t h e a p e r t u r e , n o r i n t h e

s i p h o n , n o r i n t h e o r n a m e n t , n o r i n t h e p r o c e s s o f u n c o i l i n g .

M a g n i f i c e n t e x a m p l e s o f i r r e v e r s i b f l i t y [

Y e t f r o m t h e t o t a l i t y o f s u c h a p o t h e g m s e m e r g e s a v e r y d e f in i te

a n d c o n s i s t e n t v i e w o f t h e n a t u r a l w o r l d , o f e v o l u t io n a n d o f

p a l e o n t o l o g y . D o l l o 's t h o u g h t s o n i r re v e r s i b i l i t y f l o w n a t u r a l l y ,

a l m o s t in e v i t a b ly , f r o m t h i s c o n c e p t u a l f r a m e w o r k . D i v o r c ed

f r o m i t, h i s p h r a s e s a r e e a s i l y m i s i n t e r p r e t e d . W h e n u n d e r s t o o d

b u t d e p i c te d w i t h o u t t h e th e o r e t i c a l u n d e r p i n n i n g t h a t D o l lo

p r o v i d e d , i r r e v e r s i b i l i t y a p p e a r s a s a n i s o l a t e d c u r io s i ty , a n d o n e

i s l e f t w o n d e r i n g w h y D o ll o i n v o k e d i t s o o f t e n a n d w i t h s o m u c h

a r d o r .

Dollo on the natu ral world D o l lo w a s e d u c a t e d i n t h e m e c h a n -

i st ic t r a d i t i o n t h a t d o m i n a t e d l a t e n i n e t e e n t h - c e n t u r y s c ie n c e .

H i s s t r o n g r e d u c t i o n i s t b i a s t a u g h t h i m t h a t t h e g o a l o f b i o lo g y

w a s t o a b s t r a c t f r o m t h e o r g a n i c w o r l d a s e t o f g o v e r n i n g la w s

p a t t e r n e d a f t e r t h e d e t e r m i n i s t ic s y s t e m t h e n p r e v a l e n t i n

p h y s i c s . T h i s b e l i e f n o t o n l y p r e s c r i b e d a g e n e r a l m e t h o d o l o g y

( t o s e a r c h f o r l a w s ) , b u t a l s o l e d D o l lo to a n i m p o r t a n t p a r t i c u l a r

c o n c l u s io n : t h e n e c e s s a r y a s s o c i a t io n o f a c a u s e a n d i t s e ff e c t

m e a n t t h a t a g i v e n e n v i r o n m e n t w o u l d a l w a y s e li c it t h e s a m e

t y p e o f a d a p t i v e m o r p h o l o g i c a l r e s p o n s e .

W h e n L . P l a t e c r i t ic i z e d t h e l a w o f i r r e v e r s ib f l i ty o n t h e

g r o u n d s t h a t t h e o r g a n i c w o r l d c a n n o t b e o rd e r e d a c c o r d i n g to

a b s o l u t e l y i n v i o l a b l e l a w s , D o l l o r e p l ie d : 4

I c a n n o t d e c l a r e m y s e l f to b e in a g r e e m e n t w i t h h i m , b e -

c a u s e i f t h e re a r e n a t u r a l l aw s , t h e y m u s t b e a s c o n s t a n t f o r

o r g a n i s m s a s f o r t h e in o r g a n i c w o r l d . I t s e e m s o n ly th a t t h e y

a r e m o r e c o m p l i c a t e d a n d , a s a c o n s e q u e n c e , m o r e d i f fi cu l t

t o d i s c o v e r a n d to d e f in e f o r o r g a n i s m s . T o a d m i t th e c o n t r a r y

w o u l d b e to r e t u r n t o v i t a l i s m .

A n d a f t e r b o t h h e a n d T f l ly E d i n g e r h a d w r i t t e n i n d e p e n d e n t l y

t o e a c h o t h e r o f t h e g a r d e n a t S t r a tf o r d - O n - A v o n w h e r e t h e

f l o w e r s m e n t i o n e d i n S h a k e s p e a r e ' s p l a y s a r e g r o w n ( l e t t e r o f

J u n e 4 , 1 9 2 7 ) , h e c o m m e n t e d :

3 . 1 9 2 p . 2 m .

4. 1922 p. 223.

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STEPHEN JAY GOULD

S h a k e s p e a r e G a r d e n : y e s t h e c o i n c id e n c e i s c u r i o u s B u t

t h e r e i s s o m e t h i n g e v e n m o r e c u r i o u s l I n a p u r e l y m e c h a n i c a l

e x p l a n a t i o n o f t h e u n i v e rs e , t h is c o i n c i d e n c e w a s a l r e a d y

s c h e d u l e d , b i l l i o n s o f y e a r s a g o in t h e p r i m i t i v e n e b u l a o f K a n t ,

t o o c c u r o n M a y 3 1 , 1 9 27 1 O t h e r w i s e , w h e r e w o u l d i t h a v e

c o m e f r o m ? F o r t h e r e i s n o c h a n c e in n a t u r e T h e f a m o u s

F r e n c h m a t h e m a t i c i a n L a p l a c e ( t h e s u c c e s s o r o f K a n t i n

n a t u r a l c o s m o g e n y ) s a i d t h a t " w e c a ll c h a n c e t h e p h e n o m e n a

o f w h i c h w e a r e , f o r t h e m o m e n t , i n c a p a b l e o f d is c o v e ri n g t h e

c a u s e s . " A n d a n o t h e r e x p l a n a t i o n o f th e u n i v e r s e ? I d o n o t

k n o w o f a n y .

DoUo on the nature of evolution:

T h r e e o f D o l l o ' s e v o l u t i o n a r y

v i e w s a r e p a r t i c u l a r l y r e l e v a n t t o h i s n o t i o n o f i r r e v e r s i b f l i t y :

1 . E v o l u t i o n i s d i s c o n t i n u o u s . D e V r ie s , i n f a c t , c r e d i t e d D o l l o

a s t h e fi rs t t o h a v e s t a t e d t h i s p o s t u l a t e o n t h e b a s i s o f m o d e r n

e v o l u t i o n a r y i d e a s . 5

2 . I n t h e c o u r s e o f e v o l u t i o n , d i f f e r e n t o r g a n s o f t e n e v o l v e

i n d e p e n d e n t l y o f e a c h o t h e r a n d a t d i f f e r e n t ra t e s . 6 I n a p a p e r

o n l u n g f is h e v o l u t io n D o ll o e m p h a s i z e d t h is " o v e r l a p p i n g

(chevauchement)

o f s p e c i a l iz a t i o n s " a n d c i t e d a s a n e x a m p l e : 7

Hipparion h a s p a s s e d t h e Equus s t a g e i n i t s d e n t i t i o n ; Equus

h a s p a s s e d t h e

Hipparion

s t ag e i n l im b d e v e l o p m e n t . " A l t h o u g h

h e r e t u r n e d i n f r e q u e n t l y t o th i s p r i n c i p le i n h i s p u b l i s h e d w o r k ,

h e c o n s i d e r e d it o f g r e a t i m p o r t a n c e , f o r h e l i s t e d it a l o n g w i t h

i r re v e r s ib f l i t y , d i s c o n t i n u i t y , a n d l i m i t a t i o n a m o n g " m y l a w s o f

e v o l u t i o n . " s

3 . E v o l u t i o n i s l i m i t e d . D o l l o u s e d t h i s p h r a s e i n t w o s e n s e s .

F i r st , e v o l u t i o n is l i m i t e d b e c a u s e a h i g h l y s p e c i a l i z e d o r g a n i s m

c a n n o t a d j u s t to a r a p i d ly a l t e r e d e n v i r o n m e n t a n d b e c o m e s

e x t i n c t - - a n a c c e p ta b l e s t a t e m e n t f o r m o d e r n e v o l u t io n is ts . O f

t h e s p e c i a l i z e d t u r t l e , Lytoloma, D o l l o w r o t e : 9

A l l i s s a c r i f i c e d to a n o v e r l y p r e c i s e p u r p o s e . T h e y h a v e l o s t

t h e n e c e s s a r y p l a st ic i ty t o c o n t i n u e to e v o l v e . . . A n e w p r o o f

t h a t e v o l u t i o n i s l i m i t e d , s i n c e i t i s t h e o r g a n i s m s w h o s e

s t r u c t u r e re s p o n d s m o s t e x a c t l y t o a d e t e r m i n e d a d a p t a t i o n

w h i c h d i s a p p e a r w i t h o u t d e s c e n d a n t s . . . W e s ti l l h a v e

5. 1912, 1). 140.

6. Paleontologists today refer to this phen ome no n as mosaic evolution.

7. 1895, p. 88.

8. Letter to Tilly Edinger, November 26, 1927. It is often mentioned in

subsequ ent letters.

9. 1903, pp, 25-2 6.

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Dollo on Dollo 's Law

tur t l e s , c rocod i le s , l ace r t i l i ans and even rhynchocepha l ians ,

bu t we have no more d inosaurs , i ch thyosaurs , p le s iosaurs ,

mosasaurs , o r p te rosaurs .

Second , evo lu t ion i s l imi ted because each l ineage has a de f in i te

l i fe cycle based o n an inh e re n t ly f ini t e capac i ty fo r phy le t i c

var iat ion . 10

Dol lo wrote in h is shor t paper on the laws of evolut ion:11 Is

evo lu t ion l imi ted o r indef in i t e? Does each o rg an i sm ca r ry wi th i n

i t s el f a boundl es s power o f me ta mor phos i s o r mus t i t necessa r i ly

b e c o me e x t i n c t af t e r h a v in g ru n t h ro u g h a d e t e rmi n e d c y cl e?

. . All o rga n i sm s mus t necessa r i ly become ex t inc t a f t e r hav i ng

run th ro ugh a de te rm ined cyc le which may , however , be ex-

t reme ly long. Later , DoUo suppo rted the view of h is ~mi nen t

mai t r e , A. Giard , tha t '2 iv ing foss i ls such as L i n g u l a a n d t h e

opossum have s topped evo lv ing because they have no more

d i spensab le po ten t ia l for modi f ica t ion and they would d ie ra the r

th an cha nge . 12

D o l l o o n t h e n a t u r e o f p a l e o n t o l o g y :

Dollo be l ieved tha t phy-

logeny ~l l a lways be the supr eme goal of Paleontology, la bu t

he deplored the specula t ive approach, so charac ter is t ic of la te

n in e t e e n th - c e n tu ry p a l eo n tolo g y, t h a t b u i l t l i n e a g es f ro m mo r -

phologica l ser ies wi th out rega rd to the adapt i ve s ign if icance of

observed s tages . A t ruly evol ut io nary pa leont ology could onl y

resul t f ro m the synthes is of two approaches : 14 Phylogenet i c

pa leon to logy which s tud ies inhe r i t ed cha rac te r s in o rde r to

es tab l i sh fd ia t ion and e tho log ica l pa leon to logy which s tud ies

adap t ive cha rac te rs i n o rde r to recognize convergences . Fa i lu re

to recognize conver genc e was the pr im e error of the specula t ive

school. 15 Onl y an etholog ical ap pro ach coul d correct suc h errors .

lO. Such a belief is usually associated with various shades of vitalism,

but this was certainly not the case with Dollo. Lamarck was accused of

vitali sm for his belief in the sen~ment inf~rieur but the existence of such

fluxes and/tows was central to his view of the physical world and carried

no implication of a special status for life. Likewise, Dollo believed that

phyletic life cycle was as na tu ra l an idea as indiv idu al life cycle. As a

convinced mechanist , Do]lo was a foe of vitali sm in any of its forms. Never

could any inte rna l force work to produce or even to preserve a n inadaptive

configuration. An old species dies because it canno t evolve the required

adaptation to a changing environment.

11. 1893, p 165

12. 1905a, p. 131.

13. 1909a, p 386

14. Ibid. p. 387.

15. To emphasize this point, Dollo often and proudly cited his demon-

strat ion (1895) tha t the gephyrocercal tai l of modern lungfishes is a second-

ary adaptation to benthic life and not a sign of primitive status. At that

193


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B a s i c e n v i r o n m e n t s a r e f e w ; s t a g e s i n a l i n e a g e m a n y . A t s o m e

p o i n t i n t h e h i s to r y o f m o s t l i n e a g e s , a d e r i v e d f o r m w i ll r e t u r n

to t h e e n v i r o n m e n t o f a d i s t a n t a n c e s t o r . S i n c e th i s e n v i r o n m e n t

r e q u i r e s a d e f in i te a n d p r e d i c t a b l e f u n c t i o n a l a d a p t a t i o n ( a c o n -

s e q u e n c e o f D o l lo 's d e t e r m i n i s m ) , c o n v e r g e n c e t o t h e e x t e r n a l

f o r m o f t h e a n c e s t o r m u s t o c c u r . I f p a l e o n t o l o g y c a n s u c c e e d i n

i ts s u p r e m e g o a l , t h e s e c o n v e r g e n c e s m u s t b e r e c o g n i z a b le ; th e

d e r i v e d f o r m m u s t b e d i s t i n g u i s h a b l e f r o m i t s d i s t a n t a n c e s t o r o f

t he s a m e e n v i r o n m e n t .

I t i s a t t h i s p o i n t t h a t t h e c o n c e p t o f i rr e v e r s i b i li t y e n t e r s

D o l l o ' s s y s t e m , f o r i r r e v e r s ib i l i t y p r o v i d e s t h e g u a r a n t e e t h a t

A ~t

F i g . l a . D o l l o s d e t e r m i n i s m ; t a i l s h a p e i s a s u r e g u i d e t o e n v i r o n m e n t . F o r m i s d e te l

t e r m i n e d b y m o d e o f l i fe ( p o i n te d i n b e n t h i c e n v i r o n m e n t , f a n - s h a p e d i n n e c t i c ) . F r o m let1

o f A p r i l 7 , 1 92 8 .

c o n v e r g e n t f o r m s c a n b e d i s t i n g u is h e d a n d p l a c e d i n t h e ir

p r o p e r p o s i t io n s i n a n e v o l u t i o n a r y s e q u e n c e . I r r e v e r s i b i l i t y i s n o

i s o l a te d c u r i o s i t y i n D o l l o 's th o u g h t , b u t a n e s s e n t i a l s t e p i n h i s

a r g u m e n t t h a t p a l e o n t o l o g y i s a w o r t h w h i l e e n d e a v o r .

t i m e , m a n y p a l e o n t o l o g i s t s w a n t e d t o v i e w l i v i n g l u n g f i s h a s s u r v i v o r s o f

a p r i ~ t i v e s t o c k t h a t h a d g i v e n r i s e t o t e r r e s t r i a l v e r t e b r a t e s .

1 9 4


7/24

Dollo on Dollo s Law

The definition of irreversibility is given in this context in

Dollo s work on secon dary quadr upeda lism in dinosaurs. 1 This

article begins:

In all studies of adaptation, we must distinguish with care

whether we are dealing with a primary or a secondary adapta-

tion. In other terms, whether the organism is evolving to

_~ r :ZIp _

lb . Dol lo s use of i r revers ibi li ty . T he rhipidocercal ta i l of rl~hagoriscus i s secondari ly

ned from dorsal and anal f ins af ter i r revers ible loss of the caudal f in . Redevelopment

nect ic mode of l i fe prescribes the fan-shaped tai l (determinism of f igure la) , but the

f inal rh ipidoce rcal s t ruc ture could not reappe ar exact ly due to i r revocable modif icat ions

:oduced during an in tercala ted benthic l ife. Fro m let ter of June 9 , 1928.

16. 1905b.

195


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STEPHEN JAY GOULD

s a t i s f y c e r t a i n d e t e r m i n e d c o n d i t io n s o f e x i s t e n c e f o r t h e f ir s t

t i m e , o r , h a v i n g o n c e l e f t t h e s e c o n d i t i o n s o f e x i s t e n c e , i t is

r e t u r n i n g to t h e m , a f t e r h a v i n g a d o p t e d , f o r a m o r e o r l e ss

l o n g t i m e , a n o t h e r w a y o f l if e .

H e t h e n d e f i n e s i rr e v e r s i b i l i t y a s f o ll o w s :

An organism never returns exactly to a former state eve n

if it finds itself placed in conditions of existence identical

to those in which it has previously lived. But by virtue of the

indestructibility of the past . . . it always keeps some trace

of the intermediate stages through which it has passed iv

T h o u g h D o l l o i s o f te n d o n e t h e i n j u s t i c e o f b e i n g l u m p e d w i t h

t h e o r t h o g e n e t i c i s ts , i t i s c l e a r t h a t h e n e v e r p r o p o s e d i r r e v e r s i -

b i l it y f o r a l l e v o l u t i o n a r y e v e n t s . Q u i t e t h e c o n t r a r y , f o r h e

o n l y i n v o k e d t he c o n c e p t w h e n h e n e e d e d t o s h o w t h a t c o m p l e t e

s t ru c t u r a l r e v e r sa l d id n o t a c c o m p a n y

reversal

i n f u n c t i o n o r

e n v i r o n m e n t a l p r e f e r e n c e ( s e e F ig . 1 f o r a n e x a m p l e i l l u s t ra t e d

b y D o l lo ) . H e a p p l i e d i r re v e r s ib i l i ty o n l y t o c o m p l i c a t e d m o r -

p h o l o g i e s : F u n c t i o n a l o r p h y s i o l o g i c a l r e v e r s a l o c c u r s ; s t r u c -

t u r a l o r m o r p h o l o g i c a l r e v e r s a l d o e s n o t o c c u r . 18

W i t h i n t h is t h e m e t h a t c o m p l i c a t e d s t r u c t u r e s a r e n o t r e -

e v o lv e d , D o l l o m i x e d t w o s t a t e m e n t s o f r a t h e r d i f f e r e n t s t a tu s .

1 . T h e e n t i re o r g a n i s m n e v e r r e t u r n s t o a f o r m e r s t a te : I f

o r g a n i s m s c o u l d r e a c q u i r e a n a n c e s t r a l f o r m e x a c tl y , o n ly a n

e x t r e m e l y c o m p l e t e f o ss i l r e c o r d o f g r a d u a l c h a n g e w o u l d p e r m i t

t h e a t t a i n m e n t o f D o l l o ' s s u p r e m e g o a l - - t h e e s t a b l i s h m e n t o f

p h y l o g e n y . F o r, to d e m o n s t r a t e s u c h a re v e r s a l , o n e w o u l d h a v e

t o b e a b le t o l in k t h e s e c o n d a p p e a r a n c e t h r o u g h a ll i n t e r m e d i a t e

s t a g e s to th e f o r m m o s t d i f f e r e n t f r o m b e g i n n i n g a n d e n d p o i n t s

( t o a t e r r e s t r i a l m a m m a l f o r th e w h a l e t h a t b e c a m e a f i s h ).

G i v e n t h e i m p e r f e c t i o n o f t h e g e o l o g i c a l r e c o r d 19 a n d , e s p e c i a l l y ,

D o l l o 's b e l i e f i n t h e d i s c o n t i n u i t y o f e v o l u t i o n , s u c h a l in k i n g

c o u l d n o t b e i m a g i n e d . T h i s f ir s t s t a t e m e n t i s n o t a n e m p i r i c a l

p o s t u l a t e ( i f su c h r e v e r sa l s o c c u r , w e c a n n o t d i sc e r n t h e m ) , b u t

an a priori m e t h o d o l o g i c a l a s s u m p t i o n t h a t p r e s e r v e s t h e p o s -

s ib i li ty o f e s t a b l i s h i n g p h y l o g e n i e s . S p e a k i n g o f s a w f i sh , D o l l o

w r i t e s : 2 0

S a w f is h h a v e n o t b e c o m e s h a r k s a g a in . O t h e rw i s e , h o w

w o u l d w e b e a b le to k n o w t h a t t h e y h a d o n c e b e e n d e p r e ss i -

17. I b i d . 1 0 . 443.

18. 1903 10. 32.

19. 1895 p. 88.

9 0. 1922 10. 218.

1 9 6


9/24

DoUo on DoUo's La w

form rays in a benthic l i fe in terc alate d between the pr ima ry

and secondary nect ic l i fe . We know i t because, in real i ty ,

sawfish are not sharks, but squalfform rays.

He called the notion of complet e reversibil i ty a postulate which,

unless we possessed an absolutely complete paleontological

ser ies (wh ich we are far f rom ha vin g) , would destroy al l pos-

siblity of arr i ving at phylogen ies, the su pre me goal of morphol-

ogy. 21

2. A complex par t of an organis m never returns exact ly to a

former s tate . This is a tes table s tatement about convergent

structures. 22 Dollo clai med t hat he base d his phylogenie s on this

second s ta teme nt of i r revers ibi l i ty, but th is was ra rely t rue. He

based the m on careful morphological compar i sons of a l l par ts ,

not only upon the indestruct ib le s igns of ancestry preserved in

the convergent s t ructure; a phylogeny can be es tabl ished even

if one par t rever ts exact ly to a former s tate . For e xample, he

s tates that mode rn pycnodont f ishes are ei ther deep-bodied and

adapt ed to a p lanktonic ] fie or fus i form and adapted to nektonic

life. 23 Ass umi ng that the ul t im ate ancestors of pycnodo nts were

fusiform, which of the two modern groups represents the pr imi-

t ive state? Not the fusiform species, says DoUo, because deep-

bodied pycn odonts appea r f irst in the geologic record (chronology)

and mai n ta in p r imi t ive squamat ion , t ee th, and ver tebra l co lumn

(morph olog y) . The fol lowing phylogeny is thus es tabl ished

without i r revers ibi l i ty

fus i fo rm pycnodonts

deep-bodied pycnodont s

fusiform ancestors

secondary nektonic l i fe

plankt onic l i fe

pr im ary nektonic l i fe

Now we can discern an example of i r revers ibi l i ty because the

21. 1907, p. 12,

22. It is a testable statement only if good faith is maintained n

interpreting the qualifying term complex. This term gives the statement

an 'open texture that allows an unscrupulous supporter to exclude any

event from its domain by claiming that the event was not sufficiently

complex. See Friedrich Waismann, Verifiability, in A. Flew (ed.),

Language and Logic (Oxford: Basil Blackwell, 1951). Flew claims that we

lose interest in such a hypothesis because it has suffered death by a

thousand qualifications (A. Flew,

New ssays in Philosophical Theology

[London SCM Press], pp. 96-97). With a reasonable limi t upon the term

complex , Dollo's statement is testable; if complex is used to exclude

any possible counterinstance, the statement becomes unfalsifiable.

23. 1912, pp. 108-109.

197


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STEPHEN JAY GOULD

neural spines of secondarily fusiform pycnodonts still possess

the reinforcement layers needed by their planktonic ancestors

to support the great body height.

Likewise, this s tat eme nt ca nn ot be used, as Dollo claimed, 24

to determine the direction of evolution. Chronology and not the

obviously reptili an na tu re of fish-like

I c h t h y o s a u r u s

teaches us

that the l ineage of crossopterygian fish--terrestrial repti le--

iehthyosaur did not proceed in reverse order. Were these stages

known only as an isolated structural sequence of modem forms,

we could not establish direction without some additional postu-

lates on the na tu re of evol ution ary ch ange. 25

Although we reject the more elaborate claims made by Dollo

for this second statement of irreversibility, its importance is

by no means diminished. As an affirmation that sec ondary

converge nces can be recognized morphologically by their preser-

vation of some trace of an interme diate stage, 26 this s tatem ent of

irreversibility is central to Dollo's scientific study of adaptation.

While the status of each of these two statements of irreversi-

bil ity is diff ere nt-- one statement is an unprovable but necessary

assumption, the other a testable proposition--Dollo correctly

offered the same justification for both. The unifying theme is

complexity. Precise reversal does not occur because this would

require that the organism retrace, exactly and in the same order,

an extremely large number of steps. Since the components of

an organism can evolve independently of one another (his

belief in mosaic evolution), the reacquisition of a complex

structure wil l demand nearly as many independent steps as

there are components--i t cannot be claimed that the parts of a

str uctu re are co mple x effects of a single cause. 2r The theo ry of

probability would not permit the second occurrence of such a

large series of inde pend ent events. Thus Dollo insisted, thou gh

the opposite clai m is still being made , 28 th at irreversibility is

not a mere empirical generalization from the facts of phylogeny.

Such a charge was insulting to a man who prided himself on the

deductive powers he had acquired through mathematical study.

24. 1922, p. 224, for example.

25. See 1905b, p. 442.

26. Osborn referred to irreversibility as the law of alternate adaptation.

H. F. Osborn, T h e O r i g i n a n d E v o l u t i o n of L i f e (New York: Charles

Scribner's Sons, 1917).

27. Pleiot~opy, and Gregor Mendel for that matter, were unknown when

Dollo formulated his views.

28. Branislav Petronievics, Sur la loi de l'~volution ir~versible, S c i .

P r o g . 1 3

(1918), 406-419; O. H. Schindewolf,

G r u n d f r a g e n d e r P a l ii o n to l -

o g l e (Stuttgart: Schweizbart, 1950), p. 209.

198


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Dollo on Dollo's La w

The i r revers ibi l i ty of evolut ion is not s imply an empir ic al

law resting on facts of observation, as many have believed. I t

has deeper causes which lead i t , in the last analysis, to a

question of probabil i t i es as with other na tur al laws. I n effect ,

evolut ion is a sum mati on of per fect ly determi ned individual

var iat ions in a per fect ly deter mined order . In order for i t to

be reversible, we would have to ad mit the int ervent ion of

causes exact ly inverse to those which gave r ise to the in-

divid ual vari atio ns which were the source of the f irst trans-

form ati on a nd also to their f ixation in an exactly inverse

orde r - - a c i r cumstance so complex tha t we cannot imagine

that i t has ever occurred. Otherwise, we might as well main-

tain that by throwing in to the ai r the characters necessary

for pr in t ing the I l iad , the poem would be completely composed

by the simple fall of these l i t t le metall ic blocks,z9

Dollo never state d i t quite so explicit ly, but I think i t fa ir to

infer that he based the appl icat ion of i rrevers ibf li ty upon the

posi t ion of a phen omen on in a complexi ty cont inuum. An

identi cal organis m, he stated, is less l ikely to be re-evolved th an

an identica l organ; a simple funct ion c an be reversed, a comple x

struc ture cannot, no Wh en a ph en ome no n reach es a sufficient

degree of complexity, requi r ing a sufficient numbe r of inde-

penden t s teps for i ts real izat ion, repet i t ion becomes absolutely

un im ag in ab le - - t he re are too ma ny other poss ibi l it ies, the prob-

abilit y is nil.

T

In his work on lungfish, he wrote: Notice tha t

we are not speaking here of an isolated chara cter but of an ent i re

series of ch ar ac te rs . . . Now i t i s, above all , in i ts act ion upon

highly mult ip le elements , that we can af fi rm with cer ta inty that

evol uti on is not reve rsib le. 32

As with all conti nua, there will be proble ms with borderli ne

cases, and Dollo cited as exam ple s of ir reversibfl i ty some phe-

nome na that most of us would class in the s imple, revers ible

range. 8a I would put in this category of mis pla ced b orderlin e

cases the claims that bone der ived f rom car t i lage cannot rever t

to car t i lage, and that secondar i ly mar gin al t r i lobi te eyes will

29. 1913, p. 59.

30. 1900, p. 14; 1903, p. 32.

31. 1922, p. 215.

32. 1895, p. 122.

33. It is, of course, well known that simple structures with a simple

genetic base can be reconstituted when lost. See Bjorn Kurt6n, Return of

a lost structure in the evolution of the felid dentition,

S o c . S c i e n t . F e n n

C o m m e n t . B io l . 2 6

(1963), 3-11; G. I-Iemmingsmoen, Zig-zag evolution.'

N o r s k G e ol . T i d s . 4 4 (1964), 341-352.

199


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STEPHEN JAY GOULD

n o t a s s u m e t h e i r o r i g i n a l p o s i t i o n r e l a t i v e to t h e gl ab el la . 34 T h e s e

e x a m p l e s a r e u n f o r t u n a t e , f o r t h e y l e a d t o t h e i m p l i c a t io n t h a t

D o l lo h a d i n m i n d m o r e t h a n j u s t c o m p l e x i t y a s a j u s t i f ic a t i o n

f o r i r r e v e r s i b i l i ty .

I r r e v e rs i b i l i ty , as m o s t o f t h e v e r n a c u l a r w o r d s w e b o r r o w f o r

s c i en t if i c j a r g o n , i s n o t b le s s e d w i t h t h e u n a m b i g u i t y o f a si n g l e

m e a n i n g . A s w e h a v e b e e n u s i n g t h e t e r m , i r re v e r s ib f l i t y i s a

f u n c t i o n o f t h e c o m p l e x i t y o f a s e ri e s o f i n d e p e n d e n t e v e n t s .

T h e r e i s , h o w e v e r , a n o t h e r s e n s e o f i r r e v e r s i b f li ty e x e m p l i f i e d

b y a n o t i c e I o n c e f o u n d t a c k e d t o a c o ff e e m a c h i n e : I r re -

v e r si b fl i t y : y o u c a n ' t g e t y o u r d i m e b a c k , b y p o u r i n g t h e c o f fe e

b a c k in t o t h e m a c h i n e . T h e d i m e i s l o s t n o t b e c a u s e m a n y i n de -

p e n d e n t e v e n t s a r e i n v o l v e d i n it s r e t u m ( f l ip p i n g 1 0 0 h e a d s i n

a r o w w i t h a n h o n e s t c o i n ) , b u t b e c a u s e t h e m a c h i n e i s p r o -

g r a m m e d i n s u c h a w a y t h a t o n c e t he d i m e is c o m m i t t e d , i t

c a n ' t b e r e c l a i m e d w i t h o u t b r e a k i n g t he r u l e s ( j i m m y i n g t h e

l o c k , b r i b i n g t h e c o l l e c t o r ) . T h e i r r e v e r s i b i l i t i e s p r o p o s e d b y

v a r i o u s o r t h o g e n e t i c t h e o r i e s f a l l i n t h i s s e c o n d c a t e g o r y . I f i t

w e r e t r u e t h a t s u c c e s s iv e m e m b e r s o f p h y l e t ic s e r ie s a l w a y s i n -

c r e a s e d i n s iz e , t h e n a r e v e r s e t r e n d w o u l d b e e x c l u d e d n o t b e -

c a u s e s m a l l e r s i ze i s a c o m p l e x g e n e t i c c h a n g e b u t b e c a u s e

e v o lu t io n w o u l d t h e n b e p r o g r a m m e d to p r e v e n t t h i s t e n d e n c y a s

l o n g a s t h e r u l e s w e r e f o l l o w e d .

D i d D o l lo e v e r s p e a k o f a n i r r e v e r s i b f l i t y i n t h i s s e c o n d s e n s e ?

I n p a r t i c u l a r , w e r e c e r t a i n t r e n d s i n e v o lu t io n p r o g r a m m e d i n

s u c h a w a y t h a t e m b a r k m e n t u p o n t h e m c o m m i t te d a l in e a g e

t o a d e f in i te a n d i r re v e r s ib l e c o u r s e o f d e v e l o p m e n t ? H e r e D o U o

is a m b i g u o u s . H e o f t e n w r o t e t h a t c e r t a i n s p e c i a l i z e d f o r m s

c a n n o t b e c o m e g e n e r a l i z e d , b u t t h e s e s h o r t , d e c l a m a t o r y s t a t e -

m e n t s l e a v e u s w o n d e r i n g w h e t h e r t h is i r r e v e rs i b fl i t y a r i se s

b e c a u s e s p e c i a li z e d f o r m s i n e v i ta b l y lo s e s t r u c t u r e s t oo i n t r i c a t e

to b e r e g a i n e d ( t h e u s u a l a r g u m e n t b a s e d o n c o m p l e x i t y )

o r b e c a u s e s o m e t h i n g i n h e r e n t i n t he e v o l u t io n a r y p r o c e s s

d i c t a t e s t h a t l i n e a g e s m u s t p r o c e e d f r o m g e n e r a l i z e d t o s p e c i a l -

i z e d f o r m s ( t h e o r th o g e n e t ic a r g u m e n t b a s e d o n p r o g r a m m e d

s e q u e n c e ) . R e f u t i n g , f o r e x a m p l e , t h e l i n k b e t w e e n p t y c t o d o n t

a r t h r o d i r e s a n d h o l o c e p h a l i a n s , D o U o w r o t e : I f p t y c t o d o n t s a r e

h o l o c e p h a l i a n s t h e n t h e m o s t a n c i e n t h o l o c e p h a l ia n s a r e t h e

m o s t s p e c ia l iz e d . H o w , f r o m t h is s t a t e , c o u l d t h e y g i ve r i s e to

t h e i r s u c ce s s o r s . I m p o s s i b l e . A n d h e r e i s a n e w a p p l i c a t i o n o f

t h e i r r e v e r s i b f l i t y o f e v o l u t i o n . an

34. 1909b 10. 430; 1909a p. 410.

35. 1907 p . 7.

2


13/24

D o l lo o n D o U o ' s L a w

T h e s o l u t i o n t o t h is p r o b l e m i s p r o b a b l y c o n t a i n e d i n D o U o 's

v i e w s o n t h e l i m i t a t i o n o f e v o l u ti o n , f o r h e r e w e f i n d t h e s a m e

a m b i g u i t y ( s e e p a g e 1 9 3 ) . L i m i t a t io n b a s e d o n t h e i n a b i li ty o f a

v e r y s p e c ia l iz e d f o r m t o a d a p t t o n e w c o n d i t io n s l e a d s t o a n

i r re v e r s i b il i t y d e t e r m i n e d b y t h e l os s o r i r re v o c a b l e m o d i f i c a t io n

o f c o m p l e x s t r u c tu r e s . L i m i t a t i o n b a s e d o n t h e c u r t a i l m e n t o f

p l a s t i c p o t e n t i a l i n r a c i a l o ld a g e l e a d s t o a n i r r e v e r s i b i l i t y

d e t e r m i n e d b y p r o g r a m m e d s e q ue n c e . S i n ce h e s p e a k s o f l im i t a -

t i o n i n b o t h s e n s e s , w e m u s t c o n c l u d e t h a t a n o r t h o g e n e t i c i n -

t e r p r e t a t i o n o f a f e w o f D o l lo ' s s t a t e m e n t s o n i r re v e r s ib i l i ty i s

n o t i n c o n s i s t e n t w i t h h i s v i e w o f th e e v o l u t i o n a r y p r o c e s s . T h u s ,

D o l l o m u s t s h a r e a p o r t io n o f t he b l a m e f o r t h e m o r a s s o f

c o n f u s i o n t h a t h i s l a w g e n e r a t e d i n o u r l i t e ra t u r e . A n d y e t , t h e

t e x t t h a t d e f i n e s i r re v e r s i b f l i ty a s a n a d j u n c t o f o r t h o g e n e t i c

t h e o r i e s d o e s D o l lo a g r e a t i n j u s t i c e , f o r o n l y a v e r y f e w o f h is

s t a t e m e n t s c o u l d b e f a i r l y i n t e r p r e t e d i n t h is l ig h t a n d n o n e n e e d

b e .

I n c o n c l u s i o n , t h r e e s e n s e s o f i r r e v e r si b i l i t y m a y b e d i s c e r n e d

i n D o l l o 's w o r k s :

1 A n a p r i o r i a s s u m p t i o n t h a t a w h o l e o r g a n i s m n e v e r r e v e r t s

c o m p l e t e l y to a p r i o r p h y l o g e n e t i c s ta g e .

2 . A t e s t a b le h y p o t h e s i s t h a t a c o m p l e x p a r t o f a n a n c e s t o r

n e v e r r e a p p e a r s i n e x a c t ly t h e s a m e f o r m i n a d e s c e n d a n t .

T h e s e t w o p r o p o s i t i o n s a r e t h e h e a r t o f i r r e v e r s ib f l i ty ; a l l o f

D o l lo ' s s t a t e m e n t s c a n , a n d p r o b a b l y s h o u ld , b e t i e d to o n e

o f t h e m . T h e y h a v e t h e s a m e j u s t i f i c a ti o n ( a p r o b a b i l i t y a r g u -

m e n t b a s e d o n c o m p l e x i t y ) a n d p l a y t h e s a m e ro l e ( a m e t h o d

f o r t h e r e c o g n i t i o n o f c o n v e r g e n c e s ) .

3 . C e r t a i n e v o l u t i o n a r y t r e n d s a r e n e c e s s a r i l y u n i d i r e c ti o n a l .

T h i s i n t e r p r e t a t i o n c a n b e a t t a c h e d o n l y t o a v e r y f e w o f D o l lo 's

s t a t e m e n t s . I f i n D o l l o 's m i n d a t a l l , i t p l a y e d a n e x t r e m e l y

m i n o r r o l e i n h i s t h o u g h t o n i r r e v e rs i b il i t y .

I I. T H E D E B A T E O V E R I R R E V E R S I B I L I T Y

Tis wi th ou r j udgmen t s a s ou r wa t ches , none

Go just al ike, yet e ach be l ieves his own.

A . P o p e ,

E s s a y o n C r i t i c i s m

T h e l i t e r a t u r e o n i r r e v e rs i b i l i t y i s a q u a g m i r e . I h a d h a r b o r e d

t h e n a i v e h o p e o f f i n d i n g s o m e c h r o n o l o g i c a l t r e n d i n o p i n io n ;

e i t h e r th e h a p p y d i s c ov e r y t h a t a c c u m u l a t i n g w i s d o m d r iv e s o u t

2 0 1


14/24

S T P H N J A Y G O U L D

misinterpret tion

o r th e c y n i c a l re s u l t t h a t t h e f u r t h e r w e g e t

f r o m D o U o ' s w r i t i n g , t h e l e s s li k e l y w e a r e t o r e a d i t a n d t h e

m o r e l i k el y , t h e r e f o r e , t o m i s r e p r e s e n t . I f in d , i n s t e a d , t h a t

the

t e n d e n c y t o m i s r e a d ( o r n o t r e a d ) is t im e l e s s : t h e s a m e

e r r o r s h a v e b e e n c r o p p i n g u p w i th t h e s a m e f r e q u e n c y e v e r s i n c e

t h e d e b a t e s t a r te d . I h a v e t r i e d , t h e r e f o r e , t o d e l i m i t w h a t I

c o n s i d e r t h e s ix m a j o r i n t e r p r e t a t i o n s o f D o ll o ' s o p in i o n . F o r

s e v e r a l o f t h e s e w e h a v e D o l l o ' s o w n r e f u t a t i o n o f v i e w s a t -

t r ib u t e d t o h i m , r e c a l l in g M a r x ' s f a m o u s a b j u r a t i o n o f

m a r x i s m .

A p r i m a r y d i v i s io n a r i s e s f r o m t h e t w o p r i n c ip a l m e a n i n g s

o f i r r e v e r si b i li t y : D o l l o 's b a s e d o n t h e i m p r o b a b i l i t y o f r e v e r s i o n

i n c o m p l e x s t r u c tu r e s , a n d t h e o r t h o g e n e t i c b a s e d o n t h e n e c e s -

s i t y o f f o l lo w i n g a p r o g r a m m e d s e q u e n c e .

A m o n g t h o s e w h o i n c o r r e c t l y i m p a r t e d a n o r t h o g e n e t i c i n -

t e r p r e t a t i o n t o D o l l o 's v i e w s , w e s e e t w o t e n d e n c i e s .

1. O r t h o g e n e t i c i s t s w h o c i t ed t h e i r m i s u n d e r s t a n d i n g t o s u p -

p o r t a t h e o r y o f i n e l u c t a b l e t r e n d s . D e p 6 r e t d e f i n e d D o U o 's l a w

a s t he f a c t th a t a l in e a g e , h a v i n g o n c e e m b a r k e d o n t h e p a t h

o f a d e t e r m i n e d s p e c i a li z a t i o n , c a n i n n o c a s e tu r n b a c k u p o n t h e

r o a d a l r e a d y t ra v e l le d . W e f in d h i m , a f e w p a g e s l a te r , u s i n g

t h is d e f i n it io n to s u p p o r t a c l a i m t h a t e a c h p h y l e t i c b r a n c h h a s

a ge o lo g ic c a r e e r in w h i c h w e c a n d i s t in g u i s h a p h a s e o f y o u t h ,

a p h a s e o f m a t u r i t y , a n d f in a l ly a p h a s e o f se n i li ty o r d e g e n e r a -

t i o n p r e p a r a t o r y t o e x t i n c t i o n o f th e t y p e . 36 L i k e w i s e , C u 6 n o t ,

w h o b e l ie v e d t h a t o r t h o g e n e s i s i s p r e p a r a t o r y to t h e f u t u r e o f

l i n e a g e s j u s t a s o n t o g e n e s i s i s p r e p a r a t o r y t o t he f u t u r e o f

a n i n d iv i d u a l , i n t e r p r e t e d D o l lo ' s l a w t o m e a n t h a t c e r t a i n

t r e n d s w e r e n e c e s s a r i l y u n i d i r e c t i o n a l . 37

B e u r l e n , t h o u g h n o t c o m p l e t e l y i n t h e o r t h o g e n e t i c c a m p ,

s h a r e d

the ntimech nism

o f D e p 6 r e t a n d C u 6 n o t. H e d e v o t e d

a n e n t i r e c h a p t e r t o a s u p p o s e d d e m o n s t r a t i o n t h a t t h e v a l id i ty

o f D o l lo 's l a w d e m o l i s h e s a p u r e l y m e c h a n i s t i c i n t e r p r e t a t i o n o f

t h e n a t u r a l w o r l d : T h e k n o w l e d g e t h a t p h y l o g e n e t i c h i s t o ry i s

i r re v e r s i b le m e a n s t h a t a m e c h a n i s t i c - c a u s a l in t e r p r e t a t i o n , h o w -

e v e r i t i s c o n s tr u c t e d , e i t h e r p r i m a r i l y L a m a r c k i a n o r p r i m a r i l y

D a r w i n i a n , c a n n o t a l o n e s u ff ic e f o r a n u n d e r s t a n d i n g o f p h y -

l o g e n y . as D o ll o, a c o n v i n c e d m e c h a n i s t , w o u l d h a v e t a k e n g r e a t

3 6 . C h a x l e s D e p 6 r e t , L e s T r a n s f o r m a t i o n s d u m o n d e a n i m a l P a r i s :

E r n e s t F l a m m a r i o n , 1 9 1 9) , p p . 2 4 3 , 2 4 6 .

3 7 . L . C u 6 n o t , L E v o l u t i o n b i o lo g i q u e , P a r i s : M a s s o n , 1 9 5 1 ), p p . 4 9 - 5 1 ,

5 3 7 .

3 8 . K . B e u r l e n , D i e s t a m m e s g e s c h i c h t l i c h e n G r u n d la g e n d e r A b s t a m m -

u n g s l e h r e J e n a : G u s t a v F i s c h e r , 1 9 3 7 ) , p . 4 2 .

2 0 2


15/24

Dollo on Dollo 's Law

offense a t such a use of h is work, for he had s ta ted expl ic i t ly39

that i r revers ibf l i ty offered no chal le nge to a mech ani s t i c world

view.

2. Dar win ian s who a t t ack a fa l l ac ious o r thogene t ic in te rpre -

ta t ion , o f ten wi th the ve ry a rguments DoUo used to suppor t h i s

vers i on of i r reversibi l i ty . Eas ton , 40 for ex ampl e , wrote : About

the same t ime tha t the doctr ine of or thogenes is was proposed,

a col la tera l hypothes is was sugges ted in Europe by Dol lo . I t was

DoUo' s be l i e f tha t evo lu t ion was a lways p rog res s iv e - - tha t i s, tha t

c rea tu res , once they s ta r t ed down a ce r ta in pa th , never re t rea ted

nor cou ld they re su me a fo rme r condi t ion . To und erm ine th is

supposed def ini t i on of Dol lo ' s law, E as t on c i ted the r e t ur n to

wa te r o f i ch thyosaurs and wha les , an example used o f ten by

Dol lo to subs tant ia te h is not ion of i r revers ibf l i ty .41 Similar

or thog enet ie misdef ini t ions are proposed as DoUo's own and

re jec ted by Beerbower and Ehren berg . 42

Four ma jor a rguments a re p resen ted by sc ien t i s t s who rec -

ognized a t leas t a par t of Dol lo' s c l a i m- - t ha t revers ibi l i ty of

s t ru c tur e is the objec t of debate .

3. Dollo claimed only that complex s tructures could not be

reacquire d. 4a Thi s is not alwa ys va lid beca use ances tral struc-

tures are preserve d in early ontogen etic s tages due to accelera-

t ion) and may again become adul t in paedomorphic forms .

Nopcsa a t t r ibu ted the reappearance o f the pos to rb i t a l ba r in

ma mm al s to such a process and s ta ted: 44 The unex pl a i na bl e

bu t imp or t an t fac t , t ha t the l i fe-h i sto ry o f each ind iv id ua l i s

a lways a d is tor ted recapi tu l a t i on of the his tory of it s whole

phylum, g ives the c lue by which we can unders tand why a

39. 1905a, p. 130.

40. W. H. Easton, Invertebrate Paleontology (New York: Harper, 1960),

19. 42.

41. 1912, 19. 106, and 1922, 19. 216.

42. ft. R. Beerbower, Search for the Past (Englewood Cliffs, N. J.: Prentice-

Hall, 1960), 19. 156; Kurt Ehrenberg, Paldozoologie (Vienna: Sl~ringer, 1960),

19.22.

43. The correct interpretat ion so far.

44. Franci s Nopesa, Reversible and irreversible evolution; a study

based on reptiles, Proc. ZooL Soe. London, (1923), 1058. The same 1point

has been made, with different examples in: G. J. Fej6rvary, 'Quelques

observations su~ la loi de Dollo et F619istr619hog~n~se en considerat ion

s19~eiale de la loi biog6n6tique de Haeckel, Bull. Soc. Vaud. Sci. Nat.,

53 (1920), 343-372; P. P. Sushkin, 'Notes on the 19re-Jurassic telxa19ods

from the U.S.S.R., Tray. Inst. PaldozooL Acad. Sci. U.S.S.R. Leningrad, 5,

(1936), 43-91; and, more recent ly by: ~I. A. Shishkin, Morphogenetic

factors and the irreversibility of evolution, Paleont. J., 3 (1968), 293-299.

203


16/24


l imi ted reve rsa l o f evo lu t ion can occur . " Dol lo had an t ic ipa ted

th i s ob jec t ion and prov ided a coun te r a rg ume nt based on the

fac t tha t "on togeny i s no t a comple te a nd exac t recap i t u la t io n o f

phylogeny." 45 Not only is recap i tu la t ion a lways i mper fec t (pre-

c lud in g the

exac t

reacqu is i t ion of a s t r uc tu r e ) , i t i s al so fa r f rom

unive rsa l in occur rence . Of the on togeny of the tu r t l e Dermo-

chelys coriacea DoUo wro te: 46

T h e re mu s t b e a p e r t u rb a t i o n i n r e c a p i t u l a t i o n - - s i n c e i f

we re fuse to admi t pe r tu rba t ion , we a r r ive a t a phy logeny in

opposit ion to chronology, paleontology, ethology, etc . Let us

avo id abus ing a l aw whose fa l l ac ie s have a l ready been enunc i -

a ted by the i l lus t r iou s Fr i tz Mii l le r; i t has va lue only i f we

apply i t wi th d i sc r imin a t ion .

Moreover, he wrot e to Til ly Edi nger : "It is clear, a priori t h a t

on togeny canno t be a comple te and exac t recap i tu la t i on o f phy l-

ogeny . Jus t th ink how lo ng i t would t hen t ake fo r even a s imple

ind ivi dua l to develop " ~

4. Dol lo c laimed that no s tructural reversal o f any k ind was

possible in evolution. Two mis in te rpr e ta t i ons are invo lved he re

s ince Dol lo be l ieved tha t no comple te reve rs ion o f c o m p le x

s t ruc tu res cou ld occur . Many au thors have c i t ed incomple te

revers ions of complex s t ruc tures as except ions to Dol lo ' s law.

The se condary i sodont t ee th o f ce taceans , reca l l ing the den t i t i on

of u l t i ma te rep t i l i an ances to rs , is a pe re nn i a l favor it e : S t romer

and Sco tt invoked i t and Rens ch rega rded i t a s an "unques t ion-

able e xcep tio n to Dollo 's views. 48 But Dollo ha d an swe re d the

very same ob jec t ion in 1907:

Now the evolut ion of the de nt i t io n of whales i s one of the most

beau t i fu l e xamples o f the i r revers ib fl i ty o f evo lu t ion , s ince

the secondary i sodont den t i t ion i s no t a re tu r n to the p r imi t ive

4 5 . 1 9 2 2 p . 216.

46. 1901, p. 20.

47. Letter of November 17, 1928. Dollo had enormous respect for

Haeckel despite his doubts about recapitulation. He wrote to Tilly Edinger

(letter of June 30, 1928): "I do not want you to compare me with Haeckel

. . . We exchanged publications, but I never had a personal relationship

with him. Nevertheless, a curious thing, he was interested in me and in

my work. Abel went

t o s e e

him several times and, each time, he asked:

'How is Dollo? What is Dollo doing?" "

48. E. Stromer, Lehrbueh der Paldozoologie vol. 2 Wirbeltiere (Leipzig:

B. G. Teubner, 1912), p. 288; W. B. Scott, A History of Land Mammals in

the Western Hemisphere (New York: Macmillan, 1929), p. 656; Bernhard

Rensch, Evolution Above the Species Level (New York: Columbia Uni-

versity Press, 1960), p. 124.

2 0 4


17/24

Dollo on Dollo's Law

isodont dentition but is an isodont dentition of completely

different morphology. It is infuriating that M. von Arthaber

chose to contr adi ct before he ha d unders tood. ~9

Atav isms h ave often be en cited as except ions to Dollo's law. 5

Abel 51 devote d a le ngth y cha pte r to dismi ssing this clai m by

sho~ng that atavist ic structures never produce complete re-

version. Dollo dismissed it more gently: Have you noticed my

simian atavism on this photograph (Darwin's point on the ear).

But it is only partia l and that is a beauti ful example of irreversi-

bfli ty I 52

Other authors have tried to refute Dollo by pointing out that

complete reversion occurs in simple structures. Back mutation

is oft en cited. 53 Strome r i nvoke d the size reve rsion of dwa rf

hippos and elephants; Boulenger noted that lost vertebrae have

bee n reg ain ed in several fish lineages. 5~ Most iro nic are the

statements of authors who unwitt ingly use DoUo's own formula -

tion to refute their misconception of Dollo's law:

Deeply entrenched is the conception that phylogenetic de-

velopment is irreversible (Dollo's Law). But this is so only in

certain cases, especially when we are dealing with the entire

organism or complex organs. A reversion to phylogenetically

earlier states can occur in simple structures . . . The un-

restricted use of Dollo's l a w. . , as with the similarly extreme

use of the biogenetic law, has led the study of phylog eny into

too m a n y e rrors of interpretation.~5

5. DoUo did not claim that all evolution w as irreversible but

only that lost structures could not be regained in the same form.

Pressu red by self-styled opponents of DoUo's law who bro ught

up the irrelevant examples of reversion in simple structur es cited

above, ma ny of Dollo's supporters restricted the law to a sure

thing, but emasc ulate d it in so doing. Diener, an d especially

49. 1907, p. 7.

50. Rensch, in E v o l u t i o n cites several standard examples.

51. Othenio Abel, Paliiobiologie und Stammesgeschichte (Jena: Gustav

Fischer, 1929).

52. Lette r to Tilly Edinger, Nove mbe r 21, 1926.

53. Rensch,

E v o l u t i o n

p. 124; Walt er Zi mm er ma nn , lVIethoden der

Phylogenetik, i n G. Heberer (ed.) D i e E v o l u t i o n t ie r O r g a n i s m e n (Stuttgart:

Gustav Fisc her, 1954), pp. 25-102.

54.

L e h r b u c h

p. 285; G. A. Boulenge r, L'~vo luti on est-elle r6versible?

considera tions au sujet de certains poissons, C o m p t . R e n d . A c a d . S c i . 1 6 8

(1919), 41--44.

55. Adolf Rema ne, Die Geschlehte der Tiere, i n G. Heberer (ed.) D i e

E v o l u t i o n d e r O r g a n i s m e n (Stuttgart: Gustav Fischer, 1954), p. 419.

205


18/24

S T E P H E N J A Y G O U LD

A b e l , a r e i d e n t i f i e d w i t h t h i s r e s t r i c t i o n , a n d t h e i r o p i n i o n w a s

c o p i e d b y m a n y l a t e r a u t h o r s . 56 S i m p s o n r e f e r r e d t o t h i s p r a c t i c e

a s a k i n d l y t r a d i t i o n , b u t D o l l o h i m s e l f h a d d i s a v o w e d i t : T h e

i r r e v e r s i b i l i t y o f e v o l u t i o n d o e s n o t o n l y a p p l y t o l o s t o r r e d u c e d

o r g a n s , b u t a l s o t o f u n c t i o n a l o r g a n s . 57 T h e p o i n t i s t h a t i n s o

r e s t r i c t i n g D o l l o ' s l a w , i t s e s s e n t i a l r e l a t i o n s h i p t o c o m p l e x i t y

o f s t r u c t u r e i s o b s c u r e d a n d i t s v a l u e a s a g e n e r a l s t a t e m e n t

d e s t r o y e d .

6 . D o l l o c l a i m e d o n l y t h a t c o m p l e x s t r u c t u r e s c o u l d n o t b e

r e e v o l v e d ; t h i s is a v a l i d s t a t e m e n t .

S o m e s h o r t t e x t b o o k s t a t e -

m e n t s h a v e p r e s e n t e d D o l l o ' s o w n p o s i t i o n i n a f a v o r a b l e l i g h t ; ~s

o t h e r s , u n f o r t u n a t e l y a m a j o r i t y , e x p o u n d t h e o r t h o g e n e t i c i n t e r -

p r e t a t i o n . O t h e r w o r k s , w h i c h c o n s i d e r t h e q u e s t i o n i n g r e a t e r

d e t a i l , s u p p o r t b o t h D o l l o ' s f o r m u l a t i o n a n d h i s j u s t i f i c a t i o n

b a s e d o n p r o b a b i l i t i e s . H e r e w e f i n d t h e w r i t i n g s o f S c h i n d e w o l f ,

B l u m , M u l l e r , a n d , e s p e c i a l l y , S i m p s o n . M u l l e r , a r g u i n g f r o m

g e n e t i c s , s p o k e o f t h e s h e e r s t a t i s t i c a l i m p r o b a b i l i t y , a m o u n t i n g

t o a n i m p o s s i b i l i t y , o f e v o l u t i o n e v e r a r r i v i n g a t t h e s a m e c o m -

p l e x g e n i c e n d - r e s u l t t w i c e . B l u m , f r o m a t h e r m o d y n a m i c

s t a n d p o i n t , s t a t e d t h a t t h e c h a n c e s o f r e t r a c i n g t h e s t e p s o f

e v o l u t i o n o v e r a n y d i s t a n c e b e c o m e s v a n i s h i n g l y s m a l l a s t h e

c o m p l e x i t y o f o r g a n i s m s a n d t h e i r e n v i r o n m e n t i n c r e a s e . 59

T h u s f e w c r i t i c s w h o a t t a c h e d D o l l o 's n a m e t o t h e i r d i s c u s s i o n

o f i r r e v e r s i b i l i t y c o r r e c t l y p r e s e n t e d w h a t D o l l o h i m s e l f h a d

s a id . I c a n n o t a v o i d t h e f e e l i n g t h a t t h e s e m i s s t a t e m e n t s a r i s e

f r o m u n f a m i l i a r i t y w i t h t h e i n t e l l e c t u a l c o n t e x t o f D o l l o ' s i t -

r e v e r s i b i l i t y . F o r D o l l o , a l a w o f i r r e v e r s i b f l i t y f u n c t i o n s a s a

g u a r a n t e e t h a t e o n v e r g e n c e s c a n b e r e c o g n i z e d b y p r e s e r v a t i o n

o f s o m e a n c e s t r a l s t r u c t u r e ( i n c o m p l e t e r e v e r s i o n ) . C o n v e r g e n c e

5 6. K a r l D i e n e r , P a l fi o nt o lo g ie u n d A b s t a m m u n g s l e h r e ( L e i p z i g : S a m m l .

Go s c h e n , 1 9 10 ); O t h e n i o Ab e l ,

G r u n d z i i g e d e r P a l a e o b i o l o g i e d e r

W i r b e l t i e r e

( S t u t t g a r t : S c h we i z b a r t , 1 9 1 2 ) , a n d

P a l t i o b i o l o g i e ;

f o r e x -

a m p l e , i n : W . K . G r e g o ry , O n t h e m e a n i n g a n d l i m i t s o f i r r e v e r s i b i l i t y

i n e v o l u t i o n ,

A m . N a t . , 7 0 ,

(1936) , 517-528; G. S . Car te r ,

A n i m a l E v o l u -

t io n : A S t u d y o f R e c e n t V i e w s a n d I t s C a u se s

( L o n d o n : S i d g w i c k a n d

Jackson , 1951) .

57 . G. G. S impson ,

T h e M a j o r F e a tu r e s o f E v o l u t i o n

( Ne w Yo r k :

Co lum bia Un ive r s i ty P ress , 1953) , p . 310; D ol lo , 1909a , p . 397 .

58 . A. M. Davies , A n I n t r o d u c t i o n t o P a l e o n to l o g y ( L o n d o n : T h o m a s

Murby , 1947) ; R . C . Moore , C . G. La l icker , and A. G. :F i scher , I n v e r t e b r a t e

F o s s i l s

( Ne w Yo r k : M c Gr a w - Hi l l , 19 52 ).

5 9 . O . S c h i n d e wo l f , P a l ~ io n t o l o g ie ; H . F . B l u m ,

T i m e ' s A r r o w a n d

E v o l u t i o n

( Ha r p e r T o r c h b a c k s , 1 9 62 ), p . 2 0 0 ; H . J. M u l l e r , ' R e v e r s i b i li t y

i n e v o l u t i o n c o n s i d e r e d f r o m t h e s t a n d p o i n t o f g e n e t i c s,

B i o l . R e v . , 1 4

(1939) , 27 ; G. G. S impson ,

E v o l u t i o n ,

a n d

T h i s V i e w o f L i f e

( Ne w Yo r k :

Ha r c o u r t , B r a c e a n d Wo r l d , 1 9 6 4 ) .

9 . 0 6


19/24


is the major impediment to phylogenetic interpretations; phy-

logeny is the goal of paleontology. In this context, Dollo could

scarcely have been excluding anything but

complete

reversal.

When this context is not known, criticism may be based solely

on the various vernacular senses of irreversibflity; numerous

interpretations (none of them Dollo's) will then arise. The

seemingly endless and often acrimonious debates tha t have

raged about the concept of irreversibility have almost always

been based not on substantive disagreement concerning the

course of evolution, but rather on the sheer semantic misunder-

standing generated by using one term-- Dollo 's law for a

variety of contra dictory concepts.

III. IRREVERSIBILIT YAND THE STATUS OF

EVOLUTIONARY LAWS

The nonrecurrence of experienced events must be one of the oldest

notions of the human mind, for in any real experience our sensation of

time is unidirectional and the ix'reversibility of history and of evolution

seem to be corollaries of this.

H. Blum,

Time s Arrow and Evolution,

p. 179

Our textbooks of evolution usually describe Dollo's notion of

irreversibility in conjunc tion with other supposed evolutionary

la ws -- us ua lly with Cope's law that body size tends to increase

in phyletic sequences and Williston's law that large num ber s

of sJrnflar elements tend to be reduced to fewer differently spe-

cialized units. The attempt to order phylogenetic events into

regularities sufficiently pervas ive to be ter med l aws was a pop-

ular strategy earlier in this century. It was centered on the

reductionist view that biology should be patterned on the formal

structure of the physical sciences. W. K. Gregory, a major pro-

pon ent of this strategy, held that we ought logically to begin

with the forces inside the hydrogen atom and work outward and

upwa rd through organic chemistry to man. 6o The New tonian

synthesis had produced a set of descriptive generalizations that

ordered complex results into a simpler lawful structure. Thus,

it was argued, the maturation of evolutionary biology to true

science depend ed on the discovery of lawfu l struc ture amo ng

phylogenefic events.

The laws of Cope and Williston are attempts at descriptive

60. w. K. Gregory, Basic patents in nature, Science, 78 (1933), 561-

566.

207


20/24

STE PHEN J Y GOULD

genera l iza t i on. Dol lo ' s law, on the ot her ha nd , h as a very

different s tatus . Previously, I divided DoUo's law into two state-

ment s : (1 ) The en t i re o rga n ism never re tu rns to a fo rmer s ta te .

(2 ) A complex pa r t o f an o rgan ism never re tu rns exac t ly to a

for mer s tate . The f i rst , as d iscussed on page 196, i s no s ta tem ent

about da ta a t a l l , but a necessary a p r i o r i a s s u mp t io n t h a t

phy logen ies can be e s tab l ished , g iven the na t u re o f evo lu t ion and

the fossi l record. The second seems to have the f orm of an

e mp i r i ca l l y g ro u n d e d s t a t e me n t , b u t t he a mb ig u i t y i n t ro d u c e d

by va ry i ng in te r pre ta t ions o f complex has robbed i t o f any

preci se mea nin g a nd genera l use fu lness ( see no te 22) . Almos t

a n y r e v e r s io n c a n b e e x c lu d e d f ro m i t s d o ma in b y c l a imin g

e i the r tha t the s t ruc tu res invo lved were no t su ff i cient ly complex

or tha t the gene t ic bas i s fo r an e labora te morpholog ica l change

was s imple.

I f th is second par t of Dollo 's law is not a usef ul s ta t eme nt

in a rguments about spec i f i c phy logen ies , wha t i s i t s s t a tus and

has i t any impor tance? I f we as sume tha t complex evo lu t iona ry

events genera l l y have compl ex causesf i 1 the n DoUo's law s imply

aff i rms tha t the resul ts of evolu t ion con for m to our genera l

no t i on o f h i s tory a s a s equence o f un i que phe nom ena . 62 And

Dollo ' s f ines t ins igh t was tha t he provided as h is jus t i f ica t i on

of evo lu t iona ry i r reve rs ibf l i ty the ve ry same a rg ume nt advan ced

today fo r the un iq uen ess of h i s to r ica l eve n ts - - t he s ta t is t i cal

imp ro b a b il i t y t h a t t h e i n c a l c u l a b l e n u mb e r o f i n d e p e n d e n t c o n-

f igura t ions an te cede n t to and compr i s ing any h i s to r ical even t

should ever occur twice. Thus , Dol lo ' s law is not an adj un ct of

evo lu t iona ry theory. I t is a s t a tement , f r ame d in t e rms of an imal s

and the i r evo lu tion , o f the na tu r e o f h is to ry ; o r , pu t ano the r way ,

i t i s an a f f i rma t ion o f the h i s to r ica l na tu re o f evo lu t iona ry

events . As is so often the case, we are indebted to G. G. Simpson

for th is percept ive in te rpr e ta t ion: That evolut i on is i r revers ible

is a special case of the fact that his tory does not repeat i tself .

61. The argument leveled against Dollo's law by Nopcsa and Fej~rvary

(note 44) was based on a denia l of this premise. They claimed that complex

reversions could be produced by the reasonably simple mechanism of

acceleration followed by paedomorphosis. In this sense, their argument is

potentially the strongest of any leveled against Dollo's law, but it failed

because natu re doesn't work in the way they imagined. Yet even if it did,

we could stiU preserve Dollo's law by claiming that such reversions were

not really complex and that only reversions with complex causes should

be covered by the law. This is what I mean in stating that almost any

empir ical challenge against the law can be ref~uted.

62. G. G. Simpson, T h i s V i e w o f L i f e a n d q u o t e of H. Blum i n t r o d u c i n g

this section.

2 8


21/24


The fossil record and the evolutionary sequences that it i l-

lustrates are historical in nature, and history is inherently ir-

reversible. 63

This interpr etation of Dollo's law is involved in our j ud gme nt

upon the descriptive generalizations of Cope and Wflliston and

upo n the entire enterprise of law mak ing for phylogenetie results.

Simpson has dist inguished im man en t fro m configurational

propert ies of the universe (the for mer as the unc han ging prop-

erties of matt er and ener gy and the likewise unc ha ng in g pro-

cesses and principles arising therefrom ; the latter as the

actu al state of the unive rse or of any p ar t of it at a given time. 64

Laws are framed for immanent propert ies: we are not interested

in the melting behavior of a particular ice cube but in the

properties of water in general. Physics rarely deals with the

eonfigurational; if its formal structure is lawlike, this is because

it has excluded the configurational from its domain. The error

made by reductionists who attempted to formulate laws for the

results of evolution was tha t they as sume d a similar focus for

biology and physics. But biology often deals with the configura-

tional and the search for so-called historical laws among such

properties is not a fruitful endeavor. While I agree with Watson

and Siever that there is no formal distinction between historical

and non-h istori cal science, 6~ there is a differenee of empha sis.

There are nomothetic undertones to the results of evoluti on-- the

principle of natur al selection is among th em -- an d i t is here that

our laws must be formulated. They must be based on immanent

processes that produce events, not on the events themselves. The

'qaws of Cope and Wflliston, bas ed as they are on configura-

tional properties, are not laws in the ordinary sense but descrip-

tive generalizations of low-order probability that describe some

co mm on regularities without explaining anything. Dollo's law

is not among these. Quite the contrary: since irreversibility is an

acknowledgment of the historical nature of evolutionary events

and since that very natur e precludes the formu lation of laws for

these events, Dollo's law is a particul arized st atemen t of our

reason for rejecting the approach to evolutionary biology that

led to the laws of Cope and Williston. Dollo is done an injustice

63. T h i s V i e w o f L i f e , p. 186.

64.

I b i d . ,

p. 122. Nagel makes a similar distinction between nomothetic

and ideographic properties (Ernest Nagel, The logic of historical analysis,

i n

H. Feigl and M. Brodbeck [eds.],

R e a d i n g s i n t h e P h i l o s o p h y o f S c i e n c e

[New York: Appleton-Century-Crofts, 1953], pp. 688-700).

65. R. A. Watson, Is geology different? A critical discussion of T h e

F a b r i c o f G e o l o g y , P h i l . S c i. , 3 3 , (1966), 172-185; Raymond Siever,

Science: observational, experimental, historical,

A m . S c i e n t i s t , 5 6 ,

(1968), 70--77.

209


22/24


w h e n h i s v i e w s a r e r e l e g a t e d t o t h e w a s t e b a s k e t o f a r c h a i c

s t r a t e g i e s , f o r i r r e v e r s i b f li t y i s n o o u t m o d e d h i s t o r i c a l l a w ;

r a t h e r , t h e s e a r c h f o r h i s t o r i c a l l a w s i s o u t m o d e d b e c a u s e w e

h a v e n o w r e c o g n i z e d t h e s i g n i f i c a n c e o f i r r e v e r s ib f l i ty in D o l l o 's

s e n s e .

Y e t su r e l y t h e re i s s o m e t h i n g i ro n i c h e r e , f o r D oU o h i m s e l f w a s

a r e d u c t i o n i s t a n d q u i t e c o m m i t t e d t o t h e s e a r c h f o r h i s to r i c a l

l a w s . I n f a c t , w h e n h e p r e s e n t s h i s j u s t if i c a t i o n f o r i rr e v e r s i b i li t y ,

h i s w o r d s b e t r a y t h e d e t e r m i n i s m t h a t c h a r a c t e r i z e d m o s t r e-

d u c t i o n i s t th i n k i n g p r i o r to t h e a d v e n t o f q u a n t u m p h y s i c s : T o

r e p e a t p e r f e c t l y d e t e r m i n e d i n d i v i d u a l v a r i a t i o n s i n a p e r f e c t l y

d e t e r m i n e d o r d e r . . , w e w o u l d h a v e t o a d m i t t h e i n t e rv e n t i o n

o f c a u s e s . . , i n a n e x a c t l y i n v e r s e o r d e r . 66 I d o u b t t h a t D o U o

e v e r p e r c e i v e d t h e a n t i - d e t e r m i n i s ti c i m p l i c a t i o n s o f th e l a r g e r

g e n e r a l i t y o f w h i c h h i s ir r e v e r s i b f l i t y f o r m s a s p e c i a l c a s e . A n d

y e t , i f t h e o p e r a s o f W a g n e r , w h i c h D o l lo l o v e d p e r h a p s m o r e

t h a n h i s fo s s i l s , a r e a n y g u i d e , i t i s u p o n t h e c o n t e n t o f t h e p i e c e

i t se l f a n d n o t t h e i n t e n t o f i ts a u t h o r t h a t w e m u s t j u d g e a m a n ' s

w o r k . 6 7

6 6 . 1 9 1 3 , p . 5 9 .

6 7 . I t h a n k E r n s t M a y r , D i r e c t o r o f t h e M u s e u m o f C o m p a r a t i v e Z o o l o g y ,

E v e r e t t M e n d e l s o h n , H i s t o r y o f S c i e n c e D e p a r t m e n t , H a r v a r d U n i v e r s i t y ,

a n d C a r l P u t z , P h i l o s o p h y D e p a r t m e n t , D e P a u w U n i v e r s i t y f o r t h e i r c a r e f u l

a n d e x t e n s i v e c r i t i c i s m o f t h e m a n u s c r i p t . A . S. R o m e r k i n d l y l e n t m e t h e

c o r r e s p o n d e n c e o f D o l l o a n d T i l l y E d i n g e r a n d r e g a l e d m e w i t h t h e b i t s

o f h u m a n i n t e r e s t t h a t s u b s t i t u t e a l i v i n g m a n f o r t h e a b s t r a c t i d e a s

g l e a n e d f r o m h i s p u b l i s h e d w o r k .

A P P E N D I X

T H E L A W S O F E V O L U T I O N , b y L o u is D o l lo

T r a n s l a t e d f r o m Bull. Soc. Belge Geol. Paleontol. Hydrol. 7 [ 1 8 9 3 ] , 1 6 4 -

1 6 6 ) .

I . A c c o r d i n g t o t h e b r i l l i a n t c o n c e p t i o n o f t h e i m m o r t a l C h a r l e s D a r w i n

1 8 0 9 - 1 8 8 9 , ) :

E v o l u t i o n - - t h e t r a n s f o r m a t i o n o f o r g a n i s m s - - r e s u l t s f r o m th e

f i x a t i o n o f i n d i v i d u a l u s e f u l v a r i a t i o n s b y t h e i n f l u e n c e o f n a t u r a l

s e l e c t i o n p r o v o k e d b y t h e s t r u g g l e f o r e x i s t en c e .

A l l s p e c i e s , a n i m a l o r v e g e t a b l e , w h i c h e x i s t o r h a v e e x i s t e d s i n c e

t h e a p p e a r a n c e o f l i fe o n t h e g l ob e , o w e t h e i r o r i g i n t o th i s f u n d a -

m e n t a l l a w .

I I . B u t :

1 . W h a t i s t h e c a u s e o f i n d i v i d u a l v a r i a t i o n s ?

2 1 0


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D o l l o o n D o l l o s L a w

2. Wh at i s thei r amp l i tude ? Is i t smal l ? Is it large? Is evolut ion

ex t remely s low, o r does i t occur by ra ther rap id jumps?

3. Fr om another v iewpoint , i s evolut ion revers ible? Can an org ani sm

ret urn (ei ther to tal ly or part ia l ly) to a for mer condi t ion al ready

rea li zed in the seri es o f i ts a nces to rs ?- - e i the r by re tu r n ing in

a s ingle ju mp or by pass ing, in reverse order , thro ugh the various

stages which led to i ts origin.

4 . Final ly , i s evolut ion l imi ted or indefini te? Does every organism

carry wi th in i t se lf a boundless power o f metamor phos i s , o r mus t i t

necessar i ly become ex t inc t a f t e r hav in g r un t h rough a de te rmined

cycle ?

II i . The solut ion of these quest i ons is of great impor tanc e for the biologis t,

no t s imply fo r the en ormou s in te res t whic h they o f fe r in themselves ,

but also becaus e of thei r appl icat ions.

IV. Mr. Dollo is of the opinion:

1. that evolut ion occurs by rat her rap id jump s.

2. that an o rga nis m cann ot return , e ven part ial ly , to a for mer s tate

already realized in the series of i ts ancestors.

3. tha t a l l o rgan i sms mus t necessar i ly become ext inct , a f t e r hav ing

run th rou gh a de te rmined cyc le which m ay , however , be ex t remely

long.

He expre sses th is by saying:

E v o l u t i o n i s d i s c o n t i n u o u s i r r e v e r s i b le a n d l i m i t e d .

V. The au thor then p resen ted h i s reasons fo r th ink ing tha t th i s mus t be

so. The n he c i t ed a l a rge numb er o f examples , d rawn f rom l iv ing and

foss i l animals and from l iv ing plants , to support h is v iewpoints .

VI. Mr. Dol lo is ha pp y to say that h is ideas h ave be en accepted by his

men tor , Mr. A. Giard, Prof ess eur ~t la S orbonne, and by his good

friend , Mr. P. P elseneer, Pro fes seu r ~ l ]~cole nor mal e de Gand.

He thanked these two na tu ra l i s t s fo r the cases o f d i scon t inu i ty

and i r revers ib i l ity whi ch they had so k indly commu nica t ed to h im

(Mr. Giard: crus tace ans , p lant s ; Mr. Pelseneer: mol luscs) .

He tha nked two other of h is bes t f r iends as wel l: Mr. J. Massart ,

Assis ta nt h l Ins t i tu t botani que de rUnivers i t~ de Bruxel les , who

pointed out to him many interes t ing facts related to discont inui ty and

irrevers ibi l i ty amo ng plant s ; and Mr. G. Boulenger of the Bri t i sh

Muse um who ca l l ed h i s a t t en t ion to num ero us aspec t s o f the s t ruc-

ture of l iv ing rept i les whic h have a considerable bear ing on these

quest ions .

He also ment ioned wi th sat is fact ion that Mr. L. Errera, Professeur

l Univers i t6 de Bruxel les , agrees at leas t par t ia l ly wi th his v iews.

Final ly , in conclus ion, he s tated t hat Mr. P. Hal lez, Profe sseur

la Facul t6 des Sciences de Li l le , fo l lowing his most recent s tudies on

worms , concluded tha t evo lut ion was d i scon t inuous .

VII . Mr. Dol lo was led to these general izat ions through his special ized

research on foss i l bones tha t he has been purs u ing fo r twelve years a t

the Brusse ls Museum.

He f i rs t announc ed them in h i s course a t the So lvay Ins t i tu te

(Universi ty of Brussels) (Signed lesson of November 12, 1890).

Later he re tu rn ed to them, no tab ly in Giard s B u l l e t i n (Sep tember

2 1 1


24/24

STEPHEN JAY GOULD

VIII.

IX.

X.

20,

1891) and in this Society's

Bulletin

(October 25, 1892).

The author noted with pleasure tha t h is ideas have been adopted

without reservation by Mr. A. Lameere, Professeur ~ rUniversit6 de

Bruxelles, in h is Esqui sse de la Zoologie (Brussels, 1892) an d i n the

syllabus of his Cours sur le Tra nsf orm is me (University extension;

le sso n 3; 1893).

Mr. Dollo int ends to gather together in a s mall i l lustra ted volume all

the impo rtan t cases of discontinuity, irreversibili ty, and limi ta tion

collected by h im an d his friends . 6s

Is this to say that the laws enunciated above are the only ones which

direct the evolution of organisms? Not a t a ll . There are many other

fun dam ent a l laws: Examples : the law of recapi tula t ion, the law of

nece ssar y regressi on, etc.

Short Biblio graphy of Dollo's Work on Irreversibility

'Les Lois de l' $vohiti on,

Bull. Soc. belge Gdol. Pal. Hydr.,

7 (1893), 164-166.

Sur la phylog~nle des dipneustes, Bull. Soc. belge Gdol. Pal. Hydr., 9

(1895), 79-128.

Macrurus Lecointei,

poisson abyssal nouveau, reeueill i par l 'exp~dition

antarctique belge,

Bull. Acad. r. Belg. Cl. Sc.,

(1900), 383-401.

Sur l 'origine de la tortue luth

(Dermochely s coriacea), Bull. Soc. r. Sci.

mddic, naL Bruxelles, 59

(1901), 17-40.

'Eochelone brabantica,

Tortue marine nouvelle du Bruxellien (Eocene

moyen) de la Belgique,

Bull. Aead. r. Belg. C1. Sci.,

(1903), 792-801.

Un nouvel opercule tympa niq ue de

Plioplatecarpus,

Mosasaurien plongeur,

Bull. Soc. belge Gdol. Pal. Hydr., 19 (1905a), 125-131.

Les Dinosa uri ens adapt~s ~t la vie qu adru p~de secondaire,

Bull. Soc.

belge Gdol. Pal. Hydr., 19

(1905b), 441-448.

Les Ptyctodontes sent des arthrod~res, Bull. Soc. belge G~ol. Pal. Hydr.,

21 (1907), 97-108.

La Pal~ontologie dthologique,

Bull. Soc. belge Gdol. Pal. Hydr., 23

(1909a), 377-421.

Les Poi sse ns voiliers,'

Zool. Jahrb., 27

(1909b), 419-438.

Les C~phalopodes ada pt6 s ~t la vi e nec tiq ue seco ndai res et ~t la vie

benthique tert iare ,

Zool. Jahrb. Suppl., 15

(1912), 105-140.

Podocnemius congolensis,

tortue fluviatile nouvelle du Montien (Pal~oc~ne

in~6rieur) du Congo et r~volution des cheloniens fluviatiles,

Ann. Mus.

Congo belge, C~ol. Pal. Miner., serie

3, Bas et Moyen Congo, 1 (1913),

47-65.

'Les C~phalopodes d~roul~s et l'irreversibilit~ de r~volution,

Bijdragen

tot de Dierkunde, (1922), 215-227.

68. Such a volume was never published.

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Dollos Law

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