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Show 74 DARWINISM CHAP. common species they are sure to be found ; th~t they are everywhere of considerable amount, often reachmg 20 per cent of the size of the part implicated ; and that they are to a great extent independent of each other, and thus afford almost any combination of variations that may bo nccdc.d. It must be particularly noticed that the wh~le .scnes of variation-diaarams here given (except the three whiCh Illustrate the number ~f varying individuals) in every case represent the actual amount of the variation, not on any reduced or enlarged scale but as it were life-size. Whatever number of inches or deci~als of an inch the species varies in any of its parts is marked on the diaarams, so that with the help of an ordinary divided rule or a 0 pair of compasses the vari~tion o~ the different parts can be ascertained and compared JUSt .as If the specimens themselves were before the reader, but With much greater ease. In my lectures on the Darwinian theory in Amcri?a and in this country I used diagrams constructed on. a d1ffercnt plan, equally illustrating the largo. am~u:1t of mdcpcndcn t variability, but less simple and less mtelbg1blc. Th~ present method is a modification of that used by Mr. Franc1s Galton in his researches on the theory of variability, the upper line (showing the variability of the body) in Diagram~ 4, .5' 6, an~l 13 beina laid down on the method he has used m his expenm~ nts with sweet-peas and in pedigree moth-breeding.1 I believe, after much consideration, and many tedious experiments in diagram-making, that no better method can be adopted ~or bringing before the eye, both the amount and the peculmr features of individual variability. Variations of the Ilabits of Animals. Closely connected with those v:triations of intcrrml a.rHI externa,l structure which have been already de ·cribcd, arc the changes of habits which often occur in c~rtain individn:Lls or in whole specie:, since these must Hcccssanly depend upon some correspondina chmwc in the brain or in other par ts of the 'o 'o • . organism; and as these changes arc of great Importance ~n relation to the theory of instinct, a few examples of them w1 11 be now adduced. 1 See Tmns. Entomological Society nf L ondon, 18 7, p. 24. m VARIABILITY OF SPECIES IN A STATE OF NATURE 75 The .Kea (Nestor notabilis) is a curious parrot inhabiting the moun tam ranges of the Middle Island of New Zealand. It belongs to tho family of Brush-tongued parrots, and naturally feeds on the hon~y of flowers and the insects which frequent them, together With such fruits or berries as arc found in tho r~gion. Till quito recently this comprised its whole diet but ~mce tho country it inhabits has become occupied by Euro~eans It has developed a taste for a carnivorous diet, with alarrnina results. It began by picking the sheepskins huna out to dr; or tho meat in process of being cured. About I 68 it was first observed . to attack living sheep, which had frequently b~en found. '?th raw and bleeding wounds on their backs. ~I~ce then 1t IS ~tat~d that tho bird actually burrows into the ~Ivmg s~ccp, c~tmg Its way down to tho kidneys, which form 1ts. specml dehcacy. As a natural consequence, the bird is bcmg d~stroycd as rapidly as possible, and one of the rare and curwus members of the Now Zealand fauna will no doubt shortly cease to exist. Tho case affords a remarkable instance of how tho climbing feet and powerful hooked beak developed for ?ne set of purposes can be applied to another altogether different purpose, and it also shows how little real stab~lity t~ere may be in what appear to us the most fixed habits of bfe. A somewhat similar change of diet has been recorded by the Duke of Argyll, in which a goose, reared by a golden eagle, was taught by its foster -parent to eat flesh, which it continued to do regularly and apparently with great relish.l Change of habits appears to be often a result of imitation of wh~ch ~r. Tegetmeier gives some good examples. He state~ that If pigeons are reared exclusively with small arain as wheat or barley, they will starve before eating bca~s. But when they are thus stl1rvir~ g, if a. bean-eating pigeon is put among .them, they follow 1~s example, and thcre<tfter adopt the habit. So fowls sometlmcs refuse to cat maize, but on seeing ot~ers eat it, they do the same and become excessively fond of It. Many persons have found that their yellow crocuses were eaten by sparrows, while the blnc, purple and white coloured varieties were loft untouched; hut Mr. Tegctr meier, who grows only these latter colours, found that after 1 Nattwe, vol. xix. p. 554. |