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Show 124 , 'EXUAL SELECTION: DIImS. PART rr. wing-feathers, ancl erects his ocellatcd plumes in tho right position for their full effect; or again, how the male goldfinch alternately displays his gold-be. panglcd wings, we ought not to £ el too sure that tho female does not attend to each detail of beauty. V\T e can judge, as already remarked, of choice being exerted, only from the analogy of our own minds; ancl tho mental powers of birds, jf reason be excluded, do not fundamentally differ from ours. From these various considerations we may conclude that the pairing of birds is not left to chance ; but that those males, which are best able by their various charms to plea ·e or excite tho female, are under ordinary circumstances accepted. If this be admitted, thor is not much difficulty in understanding how malo birds have gradually acquired their ornamental characters. All animals present individual differences, and as man can modify his domestieatod birds by selecting tho individuals which appear to him tho most beautiful, so tho habitual or oven occasional preference by tho female of tho more attractive males would almost certainly lead to their modification; and such modifications might in tho course of time be augmented to almost any extent, compatible with tho existence of tho species. Variability of Birds, and especially of thei?· secondary Sexual Oharacters.-Variability and inheritance are tho foundations for the work of selection. rrhat domesticated birds have varied greatly, their variations being inh ritcd, is certain. That birds in a state of nature present individual differences is admitted by every one; and that they have sometimes been modified into distinct races, is generally admittcd.31 Variations are St According to Dr. Blasius ('Ibis,' vol. ii. 1860, p. 2!)7), there arc· 425 indubitltblc species of binls which breed in Emopc, besides GO· 'l'IIAI'. XI\'. VARIABILITY. 125 or two kinds, which insensibly graduate into each other, namely, slight differences between all the members of tho same species, and more strongly-marked deviations which occur ouly occasionally. Those latter are rare with birds in a state of nature, and it is very doubtful whether they have often been preserved through selection, and then transmittc'l to succeeding gonerations.32 Nevertheless, it may be worth while to give the few cases relating chiefly to colour (simple albinism and molani m being excluded), which I have been able to collect. Mr. Gould is well known rarely to admit the existence of varieties, for he esteems very slight differences as specific; now he states 33 that near Bogota certain humming- birds belonging to the genus Oynanthus are divided into two or throe races or varieties, which differ from each other in the colouring of the tail,-" some forms, whiclt are frCql.~entJy rcgnrdcJ ftS distinct species. Of tho ]utter Blasius thinks that only ten nrc really doubtful, and that tho other fift; ought to be united with their nearest allies ; but this shews that there must bo a considerable amount of variation with some of our European birds. It is also an unsettled point with naturalists, whether several Nol'th American birds ought to be ranked as specifically distinct from the concsponcling European species. 32 'Origin of Species,' fifth edit. 186!), p. 10·.1:. I hnd always per· cci ved, that rare and s1 rongly-marked deviations of structmo, dcsorvin"' to be called monstrosities, could seldom be preserved through natural selection, and that tho preservation of even highly-beneficial vnriations would depend to a certain extent on chance. I had also fully rtpprcciatcd tho importance of mere individual differences, and this led me to insi~t so strongly on tho importance of that unconscious form of .:;election by man, which follows from the preservation of tho most valued individuals of each breed, without any intention on his part to modify the characters of tho breed. Dut until I read an able article in tho 'North British Hoviow' (March, 1867, p. 28!), et seq.), which hns been of more usc to me than any other Review, I ditl not soc how great the chances wore against tho prcsorvrttion of variations, whether l!light or strongly pronounced, occurring only in single individurtls. ~:l 'lntroduct. to the Trochilidro,' p. 102. |