OCR Text |
Show 296 DARWINISM CHAP. The term "sexual selection " must, therefore, be rcstr~cte.d to the direct results of ma.le struggle a.nd comba.t. Th~s IS really a form of natural selection, and IS a mat~er of direct ob s erva tw. n ; whi'le I·ts res· ults are as clearly .d eduCible aAs thlo s·ef of any of the other modes in which selcctwn acts. :r:c 1 this restriction of the term is needful in the case of the !ngher anima.ls it is much more so with the lower. In butterflies the weeding out by natural selection takes place to an enormous extent in the egg, larva, and pupa stat~s ; and perhaps not more than one in a hundred of the eggs hud prod1_1ces a. perfect insect which lives to breed. Here, then, the Impotence of female selection, if it exist, must be complete.; for, unless the most brilliantly coloured males arc those which procluc~ the best protected eggs, larvre, ~nd pupre, and unless .the part10ular eggs, larvre, and pupre, whic~ ~rc able to surv1vc, ~r~ tho. c which produce the most bnlhantly coloured bnttcrfhcs, any choice the female might make must be completely swa.mpcd. If on the other hand, there is this correlation between colour d~velopment and perfect. adaptation. to conditions in all stages, then this development will necessanly proceed ~)y the aget~cy of natural selection and the general laws whiCh detcrmmc the production of colour and of ornamental appendagcs.1 General Laws of Animal Coloration. The condensed account which has now been given of tho phenomena of colour in the animal worl? will sufficiently ~how the wonderful complexity and extreme mtcrest of the suhJ cct; ·while it affords an admirable illustration of the import<tncc of the arcat principle of utility, and of the effect of the theories of n~tural selection and development in giving a new interest 1 The Rev. 0. Pickard-Camhriclge, who has devoted himself to the sttl(ly of spiders, has kindly sent n.1e ~he follow~ng ext~act from a leiter, written in 1869 in which he states Jus VIews on this questiOn:- "I 1~yself doubt that particular application of the Darwinian thl•nry which attributes male peculiarities of form, structure, colour, ancl or naJtlellt to female appetency or predilection. 'rhere is, it seems io me, umlouhtc.d ly something in the male organisation of a special, anu sexual natu.re,. :'l11clt, of its own vital force, develops the remarkable male peculian tiCs so eommonly seen, and of no imaginable use to th.at sex. In as fa,r as these peculiarities show a great vital power, they pomt ~ut to us tlw finest anu strongest individuals of the sex, and sl1ow us whiCh of them woulcl mo:t certainly appropriate to themselves the best and greatest numlJer of females, and leave behind them the strongest antl greatest nu mher of x COLOURS AND ORNAM.E· TS CITARACTERISTIC OF SEX 297 to the most .familiar facts of nature. Much yet remains to be done, both 1n the observation of new facts as to the relations between the col?urs of a.nima.ls and their habits or economy, and, more especially, in the elucidation of the la.ws of ()'rowth wb ich dcte:minc changes of colour in the various group~ ; but so much 1s a.lrca.dy known that we are able, with some confidence, to formu]a,te the general principles which have brought a.bout a.ll the beauty and va.ricty of colour which everywhere delight us in our contemplation of animated nature. A brief statement of these principles will fitly conclude our exposition of the snbjcct. 1. Colour ma.y be looked upon as a necessary result of the highly complex chemical constitution of a.nima.l tis ucs and fluids. The blood, the bile, the bones, the fat, and other tissues have cha.racteristic, and often brilliant colours, which we cannot suppose to have been determined for a.ny spccia.l purpose, as colours, since they arc usually concealed. The external orga.ns, with their various appendages and integuments, would, by the same gencra.lbws, na.turally give rise to a greater va.riety of colour. 2. We find it to be the fa.ct thn.t colour increa.scs in variety and intensity as external structures and dermal appendages become more differentiated and developed. It is on scales, hair, and especially on the more highly specialised feathers, that colour is most varied and beautiful; while a.mong insects colour is most fully developed in those whose wing membranes are most expanded, and, as in the lcpidoptcm, arc clothed with highly specialised scales. Here, too, we find an ad<litional mode of colour production in transparent lamcllro or in fine surface strire which, by the laws of intcrforcncc, produce the wonderful metallic hues of so many birds and insects. progeny. And here would come in, as it appears to me, the proper application of Darwin's theory of Nat ural 'election ; for the posse:-;sors or greatest vital poweJ· being those most frefJuently prounce<l and reprodncetl, the external sig11s of it woulrl go on developing in an ever-in creasing exaggeration, only to be checked where it became really detrimental in some respect or other to the inr1ivitlual." 'l'his passage, giving tlte indepentlent vi ews of a close ohservcr - one, moreo_ver, who l1as st~Idierl the species of an extensive group of a11imals both n~ the fielcl m~d 111 the lahomtory- very nt>arly accords with lilY own eonclu ·~ons above give~l; all(l, :o far as the 111aturerl opinions of a compete 11L naturahst have any wCJght, affonl the111 au important :;upporL. |