leg is fitted for entirely different purposes from those to which the legs of other vertebrates and other insects are put.” (Quoted by Dwight in “Thoughts of a Catholic Anatomist,” p. 235.) In the analogies of convergence, therefore, we have the exact converse of the phenomenon so often encountered in connection with homology. The latter exhibits a contrast between basic identity and superficial diversity, the former a contrast between superficial convergence and fundamental divergence. Now the extreme importance of homology is manifest from the fact that the taxonomists of zoölogy and botany have found it to be 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 complete 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 subdivision into varieties or races. These morphological considerations, 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 difference 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 arbitrary, 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 assembled 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 between the organisms classified are constantly decreasing in number, while the points of difference increase apace. Hence, whereas members of the same species have very much in common, members of the same phylum have very little in common, and members of different phyla show such structural disparity that further correlation on the basis of similarities becomes impossible