HOMOLOGY AND ITS INTERPRETATION 37 



Now the extreme importance of homology is manifest from 

 the fact that the taxonomists of zoology and botany have found 

 it to bb the most satisfactory basis for a scientific classification 

 of animals and plants. In both of these sciences, organisms 

 are arranged in groups according as they possess in common 

 certain points of resemblance whereby they may be referred 

 to this, or that, general type. The resemblance is most com- 

 plete between members of the same species, which do not 

 differ from one another by any major difference, though they 

 may exhibit certain minor differences justifying their sub- 

 division into varieties or races. These morphological consid- 

 erations, however, must, in the case of an organic species, be 

 supplemented by the additional physiological criteria of perfect 

 sexual compatibility and normal viability, as we have already 

 had occasion to note in the previous chapter. When 

 organisms, though distinguished from one another by some 

 major difference, agree, notwithstanding, in the main elements 

 of structure, the several species to which they belong are 

 grouped under a common genus, and similarly genera are 

 grouped into families. A relative major difference, such as a 

 difference in the size of the teeth, suffices for the segregation 

 of a new species, while an absolute difference, such as a differ- 

 ence in the number of teeth or the possession of an additional 

 organ, suffices for the segregation of a new genus. In practice, 

 however, the classifications of systematists are often very arbi- 

 trary, and we find the latter divided into two factions, the 

 "lumpers" who wish to reduce the number of systematic groups 

 and the "splitters" who have a passion for breaking up larger 

 groups into smaller ones on the basis of tenuous differences. 

 Above the families are the orders, and they, in turn, are as- 

 sembled in still larger groups called classes, until finally we 

 reach the phyla or branches, which are the supreme categories 

 into which the plant and animal kingdoms are divided. As we 

 ascend the scale of classification, the points of resemblance be- 

 tween the organisms classified are constantly decreasing in 

 number, while the points of difference increase apace. Hence, 



