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Show 274 DARWJNJS]\[ CHAP. as types-tho two sexes ·arc nearly alike, tho male being sometimes more intensely coloured and with fewer pale markings; but in the Amcrica.n groups-represented by P. reneas, P. sesostriK, and allies-there i a. wonderful diversity, the males having a rich green or bluish patch on the fore wiu~"S, while the females ha.vc a band or spots of pmc white, 11oL always corrc. ponding iu position to the green spot of the males. There arc, however, transitional forms, by which a. complete series can be traced, from close simila.rity to grc:tt diversity of colouring between the sexes; and this may perhaps bo only an extreme example of tho intensor colour and more concentrated markings which are a very prevalent character-istic of male butterflies. There arc, in fact, many indications of a regular succession of tints in which colour development has occurred in the various groups of butterflies, from an original grayish or brownish neutral tint. Thus in tho "JEneas" group of Pa.pilios we have the patch on the upper wings yellowish in P. triopas, olivaceous in P. bolivar, bronzy-grn.y with a whi te spot in P. erlaces, more greenish and butl' in P. iphidamas, gradually changing to the fine blue of P. brissonius, and the magnificent green of P. sesostris. In like manner, the intense crimson spots of the lower wings can be traced step by step from a yellow or buff tint, which is one of the most willc~ prea~ colours in the whole order. The greater purity a1ul mtens1ty of colour seem to be usually associated with more pointed wings, indicating greater vigour and more rapid flight. Se.t-ual Selection as a supposed Cause of Colo·ur Deoelopment. . ~r. Darwin, as is well known, imputed most of the 1 nlhant colours and varied patterns of hnttcrflies' wirws to sexual ~election-that is, to a constant preference, by f~~n:de butterflies, for the more brilliant males; the colonrs thus produced being sometimes transmitted to the males alone sometimes to both sexes. This view has always seemed t~ ~nc to be unsupported by evidence, while it is also quite madequate to ~cco~nt for the facts. The only direct cvi<lencc, as set forth w~th h_Is usual fairness by Mr. Darwin himself, is opposed to his VIews. Several cntomoloo·ists assured him that, in moths, the females evince not tho l~ast choice of their x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 275 part~ers ; :1nd Dr. W al~acc of Colchester, who has largely bred the fino_ Bombyx cynthm, confirmed this statement. Among huttm:thcs, several males often pursue one female, and Mr. D~r~m says, that, unless tho female exerts a choice tho p:.urmg_ must be left to chance. But, surely, it may be the most VI~orous or most persevering male that is chosen, not nccossanly one more brightly or differently coloured and this will be true "natural selection." Butterflies have' boon noticed to prefer some coloured flowers to others ; but that does not p~ovc, or even render probable, any preference for the colour Itself, but only for flowers of certain colours on account of the more agreeable or more abundant n~ctar obtained from them. Dr. Schulte called Mr. Darwin's attention to the fact, that in the Diadcma bolina the brilliant blue colour surrounding the white spots is only visible when we look. t?wards the insect's head, and this is true of many of the 1ndesc~nt ~olours of b~ttcrflies, and probably depend. upon the direction of tho stm:c on the scales. It is suaacstcd however, that this ~isplay of ~olour will be seen bby th~ female as tho male IS approaclung her, and that it has been developed by sexual selection.1 Bnt in tho majority of cases the_ males follow the ~emalo, hovering over her in a position which would render It almost impossible for her to sec the particular colours or patterns on his upper surface ; to do so the femal~ shou~d mount higher than the male, and fly ~owar?s h1m-bemg the seeker instead of tho sought, and this IS qmte opposed to tho actual facts. I cannot therefore th~nk that this suggestion adds anything whatdver to th~ eVI~enco f?r se~ual selection of colour by female butterflies. Th1s questiOn Will, however, be again touched upon after we have considered the phenomena of sexual colour amono- tho vertebrat~ 5 Sexual Coloration of Bi1·ds. The general rule among vertebrates, as regards colour, is, for th~ two ~exes to be alike. This prevails, with only a few e::ccep~IOns, m fishes, reptiles, and mammalia ; but in birds dive~s1ty of sexual colouring is exceedingly frequent, and is, not Improbably, present in a greater or less degree in more 1 Darwin in Nature, 1880, p. 237. |