22 THE GEOLOGICAL EVIDENCES 







birds and mammals in Mesozoic time /. e., the 

 appearance first in time of the more highly organ- 

 ized group I am compelled to ask, in what respect 

 is this appearance anomalous ? What special rela- 

 tion do these two groups hold to one another and 

 to the animals that succeed them; and in accord- 

 ance with what law should it be required that the 

 order of appearance be reversed? Manifestly, only 

 if it can be shown that the line of descent of the 

 mammal passed through that of the bird; otherwise 

 the two need bear no special relation to one another. 

 What is the zoological position of the bird, and what 

 that of the mammal ? At first sight a bird appears 

 to be most sharply defined, and absolutely isolated, 

 from all other members of the great group of 

 animals. And our conception of this isolation 

 would probably have remained intact to the present 

 day were it not for the very remarkable discoveries 

 which the paleontologist has brought to light dur- 

 ing the last half-century. 



Briefly defined, a bird is a hot-blooded verte- 

 brate animal, provided with feathers to its body, 



