350 FOUNDATIONS OF BIOLOGY 



them in possessing hair, and milk glands for suckling the 

 young. This basic likeness is expressed by including all un- 

 der the CLASS Mammalia. 



The Mammals in turn are readily distinguished from Birds, 

 Reptiles, Amphibians, and Fishes (each of which forms a sepa- 

 rate class), but nevertheless are constructed on the same 

 fundamental plan, comprising a dorsal central nervous sys- 

 tem surrounded by skeletal elements forming the skull and 

 spinal column. Therefore, all are comprehended in the larger 

 group Vertebrata, in contrast with the Invertebrate groups 

 which include Hydra, Earthworm, Crayfish, etc. (See pp. 116, 

 146, 414.) The classification of the Gray Squirrel, Sciurus 

 carolinensis, (Fig. 86.) may be outlined as follows: 



SUBPHYLTJM Vertebrata. 

 CLASS Mammalia. 

 ORDER Rodentia. 

 FAMILY Sciuridae. 

 GENUS Sciurus. 



SPECIES S. carolinensis. 



This classification of the Gray Squirrel, although it 

 incidentally serves to illustrate the general method of classi- 

 fication of all organisms, is important because it places con- 

 cretely before us the fact that organisms show such funda- 

 mental similarities with obvious dissimilarities. In short, 

 the mere fact that animals and plants naturally arrange 

 themselves, as it were, in classes, orders, families, genera, 

 species, etc., raises the question of the origin of species. Is 

 special creation implying fixity of species, or is descent with 

 modification the more plausible explanation? 



The unavoidable answer is, descent with modification 

 evolution because the principle in accordance with which 

 the groups of increasing comprehensiveness are formed is 



