2j-jS Jo2irnal of Travel and Natural History 



the birds whose . habitat is the ground, as a necessity of their 

 existence and of the continuance of their species. Many species 

 make no nest whatever, and, in general, the purpose of conceahnent 

 is still more perfectly secured by an admirable adaptation of the 

 colour of the eggs to the colour of the ground. 



Mr Wallace next looks out for other physical causes as determin- 

 ing bird architecture. " Two other factors," besides the structure of 

 the bird, he specifies — one is the effect of changes in external con- 

 dition, producing corresponding changes in the form, or the material, 

 or the situation of nests ; the other is the influence of hereditary 

 habit, tending to preserve such modifications even after they have 

 ceased to be directly useful. But hereditary habit is merely another 

 form of expression for an instinct. Hereditary habit is an inborn 

 tendency to do certain things in a certain manner — a tendency 

 purely congenital and quite independent of experience or observa- 

 tion of any kind. So far, therefore, I agree with Mr Wallace, although 

 here he does not appear to agree with himself, that this is not only a 

 " factor," but the most powerful factor of all in the nest-building of 

 birds. I agree also with him that external conditions, or what he 

 calls "environment," is another factor — for the very obvious reason, 

 that both the structure and the implanted instincts of birds must be 

 correlated with the external conditions in which they are intended 

 to live. But this is an explanation only in the sense of indicating 

 a purpose which we perceive, and which we see to be actually 

 attained. It is no explanation at all in the sense of even suggest- 

 ing any instrumentality which we can understand. The reason 

 why such correlations should exist is as clear as day. The physical 

 causes by which they have been brought about are as dark as night. 



So far I have been dealing only with Mr Wallace's preliminary 

 observations ; but these are intended to prepare the ground for 

 the new theory which follows. It is necessary, therefore, to look 

 carefully to the drift of these observations, and to the direction in 

 which they are intended to lead us. 



Nest-building, then, is represented as determined — 



1. By the ordinary faculties of observation, memory, and imi- 



tation. 



2. By the organic structure of the bird itself— the shape of its 



bill, feet, &c. 



3. By hereditary habit. 



4. By " environment " or surrounding conditions. 



It will be observed that of these four factors, all, except the first, 



