THE ORIGIN OF SPECIES 351 



solely the greater or less similarity in the structural features 

 of the organisms. It its 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, genera, etc., are 

 due to changes brought about under different unknown 

 conditions, than it is to assume that each is the result of 

 a special creative act. Especially so when we realize that in 

 a very large number of cases it is difficult or impossible to 

 decide the limits of a species, owing to variations among 

 the individuals comprising it, and it is necessary to resort 

 to subspecies and varieties in classification. And further, 

 among genera, intergrading forms demand subgenera; among 

 orders, suborders; among classes, sub-classes; and so on. If 

 we admit the origin by descent with modification of the sub- 

 species and varieties, there is no logical reason for denying 

 the same origin of species, orders, and higher groups. The 

 difference is one of degree and not of kind. Before the recog- 

 nition of evolution classification was a groping after an 

 elusive ideal arrangement which naturalists felt but were 

 unable to express except in artificial form and in transcenden- 

 tal terms. Under the influence of the evolution theory classi- 

 fication became the natural expression of biological pedigrees. 



2. Comparative Anatomy 



The evidence from taxonomy is, as has just been seen, really 

 evidence from comparative anatomy, since modern classifi- 

 cations are based chiefly on anatomical characters. The 

 various groups classes, orders, families, genera, species, 

 etc. are founded not on a single difference, nor on several 

 differences, but on a large number of similarities. For in- 

 stance, the differences exhibited throughout the five classes 

 of the Vertebrates are relatively slight in comparison with 



