134 NATURE STUDIES. 



and birds have also been arranged in one chief group 

 by reason of their affinities, the quadrupeds have been 

 made to form a province by themselves. The hairy 

 nature of the body-covering, the nourishment of the 

 young by means of milk, the fact that the young are 

 born alive, and many other characters well known in 

 popular zoology, attest the distinctive nature of the 

 highest group of animals. 



But whilst these statements cannot be questioned, it 

 must not be imagined that the quadrupeds are thereby 

 entirely separated from all other animals. On the 

 contrary, they possess their own affinities with lower 

 forms, such as evolution pre-supposes, and such, 

 indeed, as that theory of nature demands. The lowest 

 mammals, to begin with, are by no means like the 

 higher quadrupeds; and it is in the lowest confines 

 of the class, as we shall presently see, that the 

 approach to lower animals is made. The warmth of 

 blood so characteristic of quadrupeds has already 

 made its appearance in the birds, and although the 

 exact origin of the mammals is yet a matter of doubt, 

 it seems pretty clear that the root-stock of the class 

 to which man himself belongs, may be sought for in 

 some common territory whence, from a half-bird type, 

 the lowest quadrupeds arose, or whence the mammals 

 on the one side, and birds and reptiles on the other, 

 have independently arisen. Such a conclusion seems 

 to be that at present supported by facts as they 

 stand ; and although further research may modify this 

 view, there will still exist the demand for the links 



