BIRDS CLASSED FROM THE STRUCTURE OF THEIR TOES. 207 



down a stick when nearly perpendicular, and especially not 

 on the under side of one leaning over. 



But there is food, and abundant food, for birds, that can 

 be had only by perching and walking in the manner to which 

 the birds noticed are not fitted. There are insects in the 

 crevices of the boles and branches of trees adequate to the 

 supply of many birds ; and insects, too, which would do 

 serious mischief to the trees, if there were not birds to thin 

 their numbers. These birds require a peculiar structure of 

 feet ; and though they have other peculiarities for enabling 

 them to filch the insects out of their lurking-places, yet the 

 feet which enable them to get to those places, and remain at 

 them till they catch the insects, are their proper distinguish- 

 ing characters. 



We have to do only with the mere external appearances of 

 these feet ; but they are wonderful pieces of mechanism, and 

 show with how apparently little difference from that which 

 answers one purpose, a very different purpose may be effected. 



There are two distinct forms of these feet, the one called 

 zygodactylic, or " yoke-toed," and the other anisodactylic, or 

 " unequal-toed ;" and on these are founded two orders of 

 birds which agree in several of their habits, but differ in so 

 many others, as to warrant the distinction. The following 

 cut will illustrate both : 



Wood-pecker. Nuthatch. 



