Darwin c the Descent of Man and Selection in Relation to Sex

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412 SEXUAL SELECTION. PART II.

by Mr. Wallace26 in the Malayan region, and byMr. Trimen in South Africa.

As some writers27 have felt much difficulty in under-standing how the first steps in the process of mimickrycould have been effected through natural selection, itmay be well to remark that the process probably hasnever commenced with forms widely dissimilar in colour.But with two species moderately like each other, theclosest resemblance if beneficial to either form couldreadily be thus gained; and if the imitated form wassubsequently and gradually modified through sexualselection or any other means, the imitating form wouldbe led along the same track, and thus be modified toalmost any extent, so that it might ultimately assumean appearance or colouring wholly unlike that of the

other members of the group to which it belonged. Asextremely slight variations in colour would not in manycases suffice to render a species so like another pro-tected species as to lead to its preservation, it shouldbe remembered that many species of Lepidoptera areliable to considerable and abrupt variations in colour.A few instances have been given in this chapter; but

under this point of view Mr. Bates' original paper onmimickry, as well as Mr. Wallace's papers, should beconsulted.

In the foregoing cases both sexes of the imitatingspecies resemble the imitated; but occasionally the

26 ' Transact. Linn. Soc.' vol. xxv. 1865, p. 1; also ' Transact. Ent.

Soc.' vol. iv. (3rd series), 1867, p. 301.27 See an ingenious article entitled, " Difficulties of the Theory of Natural Selection," in the ' Month,' 1869. The writer strangely sup-poses that I attribute the variations in colour of the Lepidoptera, bywhich certain species belonging to distinct families have come toresemble others, to reversion to a common progenitor; but there is nomore reason to attribute these variations to reversion than in the caseof any ordinary variation.

CHAP. XVI. THE YOUNG LIKE THE ADULT FEMALES. 199

that subsequently, the females from the danger in-curred during incubation, and the young from beinginexperienced, had been rendered dull as a protection.But this view is not supported by any evidence, and isnot probable; for we thus in imagination expose duringpast times the females and the young to danger, fromwhich it has subsequently been necessary to shield theirmodified descendants. We have, also, to reduce, through

a gradual process of selection, the females and the youngto almost exactly the same tints and markings, and totransmit them to the corresponding sex and period of life. It is also a somewhat strange fact, on the suppo-sition that the females and the young have partakenduring each stage of the process of modification of atendency to be as brightly coloured as the males, that the

females have never been rendered dull-coloured withoutthe young participating in the same change; for thereare no instances, as far as I can discover, of species with thefemales dull-coloured and the young bright-coloured. Apartial exception, however, is offered by the young of cer-tain woodpeckers, for they have " the whole upper part'' of the head tinged with red," which afterwards either

decreases into a mere circular red line in the adults of both sexes, or quite disappears in the adult females.12

Finally, with respect to our present class of cases,the most probable view appears to be that successivevariations in brightness or in other ornamental charac-ters, occurring in the males at a rather late period of life have alone been preserved; and that most or all

of these variations owing to the late period of life atwhich they appeared, have been from the first trans-mitted only to the adult male offspring. Any varia-

12 Audubon, ' Ornith. Biography,' vol. i. p. 193. Macgillivray, ' Hist.Brit. Birds,' vol. iii. p. 85. See also the case before given of Indopicuscarlotta.

202 SEXUAL SELECTION: BIRDS. PART II.

tions made in China by Mr. Swinhoe.15 Mr. Blythbelieves, that the young of both sexes resemble theadult male.

The females of the three species of Painted Snipes(Rhynchæa) " are not only larger, but much more richly

" coloured than the males."16 With all other birds, inwhich the trachea differs in structure in the two sexes

15 Jerdon, ' Birds of India,' vol. iii. p. 596. Mr. Swinhoe, in ' Ibis,'1865, p. 542; 1866, p. 131, 405.

16 Jerdon, ' Birds of India,' vol. iii. p. 677.

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17 Gould's ' Handbook of the Birds of Australia,' vol. ii. p. 275.18 ' The Indian Field,' Sept. 1858, p. 3. 19 ' Ibis,' 1866, p. 298.

CHAP. XVI. THE YOUNG LIKE THE ADULT MALES. 203

it is more developed and complex in the male than in

the female; but in the Rhynchæa Australis it is simplein the male, whilst in the female it makes four distinctconvolutions before entering the lungs.17 The femaletherefore of this species has acquired an eminentlymasculine character. Mr. Blyth ascertained, by exa-mining many specimens, that the trachea is not con-voluted in either sex of R. Bengalensis, which species

so closely resembles R. Australis that it can hardly bedistinguished except by its shorter toes. This fact isanother striking instance of the law that secondarysexual characters are often widely different in closely-allied forms; though it is a very rare circumstancewhen such differences relate to the female sex. Theyoung of both sexes of R. Bengalensis in their first

plumage are said to resemble the mature male.18

There is also reason to believe that the male undertakesthe duty of incubation, for Mr. Swinhoe19 found thefemales before the close of the summer associated inflocks, as occurs with the females of the Turnix.

The females of Phalaropus fulicarius and P. hyperbo-reus are larger, and in their summer plumage " more gaily

" attired than the males." But the difference in colourbetween the sexes is far from conspicuous. The malealone of P . fulicarius undertakes, according to ProfessorSteenstrup, the duty of incubation, as is likewise shewnby the state of his breast-feathers during the breeding-season. The female of the dotterel plover ( Eudromiasmorinellus ) is larger than the male, and has the red

and black tints on the lower surface, the white crescenton the breast, and the stripes over the eyes, morestrongly pronounced. The male also takes at least a

204 SEXUAL SELECTION: BIRDS. PART II.

share in hatching the eggs; but the female likewiseattends to the young. 20 I have not been able to dis-cover whether with these species the young resemble theadult males more closely than the adult females; forthe comparison is somewhat difficult to make on accountof the double moult.

Turning now to the Ostrich order: the male of the com-mon cassowary (Casuarius galeatus ) would be thought

by any one to be the female, from his smaller size andfrom the appendages and naked skin about his headbeing much less brightly coloured; and I am informedby Mr. Bartlett that in the Zoological Gardens it iscertainly the male alone who sits on the eggs and takescare of the young.21 The female is said by Mr. T. W.Wood 22 to exhibit during the breeding-season a most

pugnacious disposition; and her wattles then becomeenlarged and more brilliantly coloured. So again thefemale of one of the emus ( Dromœus irroratus ) is con-siderably larger than the male, and she possesses aslight top-knot, but is otherwise undistinguishable inplumage. She appears, however, " to have greaterpower, when angry " or otherwise excited, of erecting,

" like a turkey-cock, the feathers of her neck and20 For these several statements, see Mr. Gould's ' Birds of Great

Britain.' Prof. Newton informs me that he has long been convinced,from his own observations and from those of others, that the males of the above-named species take either the whole or a large share of the-duties of incubation, and that they "shew much greater devotion" towards their young, when in danger, than do the females." So it is,

as he informs me, with Limosa lapponica and some few other Waders,in which the females are larger and have more strongly contrastedcolours than the males.

21 The natives of Ceram (Wallace, ' Malay Archipelago,' vol. ii. p.150) assert that the male and female sit alternately on the eggs; butthis assertion, as Mr. Bartlett thinks, may be accounted for by thefemale visiting the nest to lay her eggs.

22 'The Student,' April, 1870, p. 124.

CHAP. XVI. SUMMARY. 235

during a lengthened period have produced some definiteeffect on both sexes, the more important result will havebeen an increased tendency to fluctuating variability orto augmented individual differences; and such differ-ences will have afforded an excellent groundwork forthe action of sexual selection.

The laws of inheritance, irrespectively of selection,appear to have determined whether the characters ac-

quired by the males for the sake of ornament, for pro-ducing various sounds, and for fighting together, havebeen transmitted to the males alone or to both sexes,either permanently or periodically during certain sea-sons of the year. Why various characters should some-times have been transmitted in one way and sometimesin another is, in most cases, not known; but the period

of variability seems often to have been the determiningcause. When the two sexes have inherited all charac-ters in common they necessarily resemble each other;but as the successive variations may be differently trans-mitted, every possible gradation may be found, evenwithin the same genus, from the closest similarity tothe widest dissimilarity between the sexes. With many

closely-allied species, following nearly the same habitsof life, the males have come to differ from each otherchiefly through the action of sexual selection; whilstthe females have come to differ chiefly from partakingin a greater or lesser degree of the characters thusacquired by the males. The effects, moreover, of thedefinite action of the conditions of life, will not have

been masked in the females, as in the case of the males,by the accumulation through sexual selection of strongly-pronounced colours and other ornaments. The indi-viduals of both sexes, however affected, will have beenkept at each successive period nearly uniform by thefree intercrossing of many individuals.

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