DESCENT WITH CHANGE 353 



the Rodents, still agree with them in possessing hair, and milk 

 glands for suckling the young. This basic likeness is expressed by 

 including all under the class Mammalia. 



The Mammals in turn are readily distinguished from Birds, 

 Reptiles, Amphibians, and Fishes (each of which forms a separate 

 class), but nevertheless are constructed on the same basic plan, 

 comprising a dorsal central nervous system surrounded by skeletal 

 elements forming the skull and vertebral column. Therefore, all are 

 comprehended in the larger group Vertebrata which, with certain 

 minor groups, comprises the phylum Chordata and stands in con- 

 trast with all the Invertebrate phyla which include Hydra, Earth- 

 worm, Crayfish, etc. The classification of the Gray Squirrel, 

 Sciurus carolinensis (Fig. 108), may be outlined as follows: 



Kingdom — Animalia 

 Subkingdom — Metazoa 

 Phylum — Chordata 



Subphylum — Vertebrata 

 Class — Mammalia 

 Order — Rodentia 

 Family — Sciuridae 

 Genus — Sciurus 

 Species — carolinensis. 



This classification of the Gray Squirrel, although it incidentally 

 serves to illustrate the general method of classification of all or- 

 ganisms, is important because it places concretely before us the 

 fact that organisms show such fundamental similarities with ob- 

 vious 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 change the more plausible explanation? (See Appendix: 

 Classification.) 



The unavoidable answer is descent with change — evolution — 

 because the principle in accordance with which the groups of 

 increasing comprehensiveness are formed is solely the greater or 

 less similarity in the structural features of the organisms. It is 

 much more reasonable to assume that the thread of fundamental 

 similarity which runs through all the Vertebrates, for instance, is 

 the result of inheritance, while the differences of orders, families, 



