CLASSIFICATION AND EVOLUTION 521 



(1) It is supported by the facts upon which classification 

 is based. Species, genera, families, orders, etc., are like 

 the branches of a genealogical tree, and when they are 

 arranged as such suggest strongly that they have arisen 

 by modification each from the preceding grade. By the 

 alteration in different directions of groups of members 

 of a single species, the several species of a genus would 

 arise. As each of these pursued its own line of evolu- 

 tion it would become more unlike its congeners until 

 it reached the rank of a genus, by which time it would 

 generally have given rise to species of its own, and so 

 forth. Every attempt to classify animals results in an 

 arrangement which to some extent suggests the evolution 

 of its members, but in modern zoology classifications are 

 expressly so constructed as to show what are believed to 

 have been the lines of evolution which animals have 

 followed. Each of the groups of such a classification 

 represents an original species, from which all the sub- 

 divisions of the group are supposed to have arisen by 

 descent with modification in various directions. As an 

 illustration of this, the several groups of the classification 

 of the Mammalia given in Chapter XXI. may be arranged 

 in the form of a genealogical tree as on page 522. 



(2) The facts of morphology also support the theory of 

 evolution. In our survey of a series of types of animals 

 we have seen how organs which serve different functions 

 are often built upon the same general plan, which is 

 modified in different directions in the several instances. 

 The hands of a frog, a rabbit, a man, a horse, and a bird 

 are all built upon one plan, though the parts are of differ- 

 ent shapes in the several cases. It is difficult to find 

 any satisfactory explanation of this except evolution. Organs 

 which are believed to have arisen by modification of 

 identical organs in an ancestral animal are said to be 

 homologous?- Thus the wings of a bird and a bat are 



1 The terra is extended to include the case of members of a series in 

 one individual, such as the nephridia of an earthworm or the limbs of a 

 crayfish, which are said to be serially homologous because they are built 

 upon the same plan, so that the repetition of structure which is seen in 

 them appears to be of the same nature as the repetition of the structure 

 of an ancestor in its descendants. 



34 



