LIMITS OF NATURAL SELECTION. 19 



and yet, throughout its whole performance, may 

 display intelligence in its details. It has been ob- 

 served that young birds do not build their nests as 

 perfectly as older birds. The later improvement in 

 building such a complicated structure as a bird's 

 nest cannot be due wholly to the experience — in 

 the ordinary sense of the word — of an animal which 

 has not sufficient intelligence to loosen a slip-knot 

 tied around its leg. The old bird has, however, 

 acquired a greater general intelligence, which enables 

 its instinct to find more perfect expression. We 

 cannot doubt that it is instinct which determines the 

 general plan of a bird's nest, — its architecture and 

 the materials used ; and yet the selection of a par- 

 ticular site for building, and the adaptation of the 

 structure to the peculiar necessities of the location, 

 we must believe to depend on intelligence. For the 

 instincts of last year's birds could not have been so 

 perfectly adapted to the requirements of this year's 

 branches, as to account for the adaptation of nest to 

 branch, that we find existing. If the entire matter of 

 selecting a site and building a nest were dependent 

 purely on instinct, it would be impossible to conceive 

 how each pair of birds found the particular shape 

 of branch demanded by the separate instincts of each 

 one of them ; and many birds would, for this reason, 

 die without posterity. We must believe that intelli- 



