ZOOGENESIS 



birds the embryo develops within a rigid envelope 

 permitting but little deviation from the general type 

 represented by the parents. Furthermore the young, 

 dependent on the ministrations of one or both the 

 parents, must be of such a nature as to be able to 

 receive and to profit by parental care, and also to 

 stimulate it. This still further restricts the pos- 

 sibility of wide deviation from a general type. 



As a class the birds, especially the smaller birds, 

 are the most constantly active of all the vertebrates. 

 They have a fixed body temperature which is always 

 high, in the smaller birds more than ten degrees above 

 our own. 



All of the birds, both fossil and living, excepting 

 for the ancient toothed birds of the Cretaceous period 

 and the two genera included in the old term Archa- 

 opteryx, are very closely allied, and in spite of the vast 

 range in size from Princess Helen's hummingbird to 

 the ostrich the birds form a much more unified group 

 than do the mammals, reptiles, amphibians or fishes. 



Like the birds, the turtles and the crocodilians show 

 a remarkable uniformity of structure, a corresponding 

 uniformity in their early stages, and a similar curious 

 zoological isolation from all related types. In agree- 

 ment with the birds, there are no viviparous crocodili- 

 ans or turtles. The eggs of turtles may have a hard 

 and brittle shell, like the eggs of birds and croco- 

 dilians, or the shell may be tough and parchment- 

 like. When compared with the eggs of birds the eggs 

 of turtles and of crocodilians are small, but they are 

 much more numerous. They are carefully placed by 



