vi A THEORY OF BIRDS' NESTS 121 



their elders, without any regard to the use or applicability of 

 the particular acts. So, in savages, many customs peculiar to 

 each tribe are handed down from father to son merely by the 

 force of habit, and are continued long after the purpose which 

 they originally served has ceased to exist. With these and a 

 hundred similar facts everywhere around us, we may fairly 

 impute much of what we cannot understand in the details of 

 Bird- Architecture to an analogous cause. If we do not do so, 

 we must assume either that birds are guided in every action 

 by pure reason to a far greater extent than men are, or that 

 an infallible instinct leads them to the same result by a 

 different road. The first theory has never, that I am aware 

 of, been maintained by any author, and I have already shown 

 that the second, although constantly assumed, has never been 

 proved, and that a large body of facts is entirely opposed to 

 it. One of my critics has, indeed, maintained that I admit 

 " instinct " under the term " hereditary habit " ; but the 

 whole course of my argument shows that I do not do so. 

 Hereditary habit is, indeed, the same as instinct when the 

 term is applied to some simple action dependent upon a 

 peculiarity of structure which is hereditary ; as when the 

 descendants of tumbler pigeons tumble, and the descendants 

 of pouter pigeons pout. In the present case, however, I 

 compare it strictly to the hereditary, or more properly, per- 

 sistent or imitative, habits of savages, in building their 

 houses as their fathers did. Imitation is a lower faculty 

 than invention. Children and savages imitate before 

 they originate ; birds, as well as all other animals, do the 

 same. 



The preceding observations are intended to show that the 

 exact mode of nidification of each species of bird is probably 

 the result of a variety of causes, which have been continually 

 inducing changes in accordance with changed organic or 

 physical conditions. The most important of these causes 

 seem to be, in the first place, the structure of the species, 

 and, in the second, its environment or conditions of existence. 

 Now, we know that every one of the characters or conditions 

 included under these two heads is variable. We have seen 

 that, on the large scale, the main features of the nest built by 

 each group of birds bears a relation to the organic structure 



