354 ANIMAL BIOLOGY 



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. Again, among genera, intergrading 

 forms demand subgenera; among orders, suborders; among 

 classes, subclasses ; and so on. If we admit the origin by descent 

 with change of the subspecies 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. Be- 

 fore the recognition of evolution, classification was a groping after 

 an elusive ideal arrangement which naturalists felt but were un- 

 able to express except in artificial form and in transcendental terms. 

 Under the influence of the evolution theory, classification became 

 the natural expression of biological pedigrees. (Figs. 236, 297.) 



2. Comparative Anatomy 



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

 evidence from comparative anatomy, since modern classifications 

 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 instance, the differences exhibited 

 throughout the five classes of the Vertebrates are relatively slight 

 in comparison with the basic resemblances. This similarity in 

 dissimilarity is brought out by the science of comparative anatomy. 

 A few concrete examples, some of which we are already familiar 

 with, will serve to bring the main facts clearly before us. 



The fore legs of Frogs and Lizards, the wings of Birds, the fore 

 legs of the Horse, and the arms of Man are built on the same basic 

 plan. The same is true of the hind limbs. Clearly all are homol- 

 ogous structures, such variations as exist being brought about 

 chiefly by the transformation or absence of one part or another. In 

 short, all the chief parts of both the fore limbs and the hind limbs 

 are homologous throughout the series. All are composed of the same 

 fundamental materials disposed in practically the same way — 

 nearly all the bones, muscles, blood vessels, and nerves are homol- 

 ogous. Or compare the digestive systems of the same forms, or 



