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vi A THEORY OF BIRDS' NESTS 137of colour that this protection can be most readily obtained,since no other character is subject to such numerous andrapid variations. The case I have now endeavoured to illustrateis exactly analogous to what occurs among butterflies.As a general rule, the female butterfly is of dull and inconspicuouscolours, even when the male is most gorgeouslyarrayed ;but when the species is protected from attack by adisagreeable odour and taste, as in the Heliconidse, Danaidaeand Acrseidae, both sexes display the same or equally brillianthues.Among the species which gain a protection by imitatingthese, the very weak and slow-flying Leptalides resemblethem in both sexes, because both sexes alike require protection,while in the more active and strong-winged generaPapilio, Pieris, and Diadema it is generally the females onlythat mimic the protected groups, and in doing so often becomeactually more gay and more conspicuous than the males, thusreversing the usual and in fact almost universal characters ofthe sexes. So, in the wonderful Eastern leaf-insects of thegenus Phyllium, it is the female only that so marvellouslyimitates a green leaf ;and in all these cases the difference canbe traced to the greater need of protection for the female, onwhose continued existence, while depositing her eggs, thesafety of the race depends. In Mammalia and in reptiles,however brilliant the colour may be, there is rarely any differencebetween that of the sexes, because the female is notnecessarily more exposed to attack than the male. It may, Ithink, be looked upon as a confirmation of this view, that nosingle case is known either in the above-named generaPapilio, Pieris, and Diadema or in any other butterfly, of amale alone mimicking one of the Danaidse or HeliconidaaYet the necessary colour is far more abundant in the males,and variations always seem ready for any useful purpose.This seems to depend on the general law that each speciesand each sex can only be modified just as far as is absolutelynecessary for it to maintain itself in the struggle for existence,not a step further. A male insect byits structure and habitsis less exposed to danger, and also requires less protection,than the female. It cannot, therefore, alone acquire anyfurther protection through the agency of natural selection.But the female requires some extra protection, to balance the