On Mr Wallace's Theory of Birds' Nests 283 



there is an obvious and satisfactory reply. There is a reason, and 

 a conclusive reason, why the female bird in some species should 

 be dull coloured. Take the case of the common Pheasant, which 

 is a type of a large class similarly conditioned. If the hen Phea- 

 sant, which lays its eggs upon the ground, were as brilliantly 

 coloured as the male, she would be so conspicuous an object to 

 every predatory animal that the species would be speedily exter- 

 nainated. This, therefore, is a sufficient reason why she should be 

 dull coloured, and totally dissimilar from the male. But as regards 

 the second question, by what physical cause it is brought about, 

 that this good reason should be met and complied with, no answer 

 can be given. Mr Wallace himself makes an observation which 

 effectually disposes of the one " law," in the sense of a physical 

 connexion of cause and effect, which would most naturally suggest 

 itself The observation I refer to is this : — That the fact of both 

 sexes being equally brilliant in many species shews that there is 

 no absolute and physical connexion between the male sex in 

 birds and a brilliant plumage. All, therefore, that can be said 

 is, that it is one of those special adaptations for special purposes 

 which are so abundant in nature, but which leave us absolutely in 

 the dark as to the physical agencies by which they are secured. 



And if this be the object, or the reason, or the "final cause," of 

 the dull colouring in the female in Pheasants, it is extremely pro- 

 bable that the dull colouring which prevails in the females of other 

 species is connected with the same great purpose ; and it is equally 

 probable that when brilliant colouring is given to female birds, the 

 circumstances and conditions of their nesting are such as to dis- 

 pense with this particular protection, which is so necessary under 

 other circumstances and other conditions. 



Accordingly, Mr Wallace announces the law, or the general- 

 ized expression of the facts in this matter, to be as follows : — " That 

 when both sexes are of strikingly gay or conspicuous colours, the 

 nest is of the second class, or such as to conceal the sitting bird ; 

 while, whenever there is a striking contrast of colours, the male 

 being gay and conspicuous, the female dull and obscure, the nest 

 is open, and the sitting bird exposed to view." 



Such is Mr Wallace's statement of the facts — and now we have 

 his explanation of the physical cause. " The mode in which this 

 has been effected is very intelligible, if we admit the action of 

 natural and sexual selection. It would appear from the numerous 

 cases in which both sexes are adorned with equally brilliant 



