EVOLUTION AS SEEN IN THE CLASSIFICATION OF ANIMALS 315 



acters are, in the opinion of a competent systematist, sufficiently definite to 

 entitle it, or them, to a specific name." We note in this definition entire em- 

 phasis upon "distinctive morphological characters." We also note a point 

 mentioned in our earlier discussion — the great amount of individual subjec- 

 tive judgment which admittedly enters into decisions as to what constitutes 

 "sufficiently definite" morphological differences. 



The second definition is that of Mayr ( 1942 ) quoted earlier: "Species are 

 groups of actually or potentially interbreeding natural populations, which 

 are reproductively isolated from other such groups." This definition places 

 complete emphasis upon reproductive isolation, since the latter is felt to be 

 primary. Given reproductive isolation, "distinctive morphological char- 

 acters" will usually arise in due course (pp. 487-489). 



These two definitions represent the extremes of a series. Many writers on 

 the subject have proposed definitions of the word "species," most of them 

 combining the two attributes of structural difference and reproductive isola- 

 tion in various manners and degrees. Instead of proposing a definition of 

 our own we shall discuss the various attributes which usually characterize 

 species. 



At the outset we may note that it is by no means necessary to assume that 

 all species have "actually" the same properties or attributes, quite aside from 

 differing human judgments in the matter. Among the one-celled animals 

 (Protozoa), for example, "the species" may well be quite a differently con- 

 stituted unit from "the species" among mammals. Indeed, it could hardly 

 be otherwise. Despite differences, however, species do in general have some 

 attributes in common. Accordingly we shall list some of the attributes which 

 on the average are found to be characteristic of the species as a unit both 

 of classification and of evolution. 



In the first place, a species is a group of animals all of which usually pos- 

 sess some distinctive characteristic or characteristics. The characteristic is 

 usually morphological, a visible structure, although some species are known 

 which are not visibly distinguishable from each other. In such cases the 

 "distinctive characteristic" is chemical or physiological. When, as in most 

 instances, the distinctive characteristics are structural, the difference in struc- 

 ture between two closely allied species need not be great. Indeed, the differ- 

 ence is frequently quite small. The Florida tree snails, popular with many 

 people because of the bright colors and varied patterns of the shells, may 

 afford us an example here. These ornate snails all belong to one genus, Lig- 

 uus, divided into several species. Two of the principal species are distin- 

 guished from each other by the fact that the apex of the coiled shell of one 

 species is pink in color, while the apex of the shell of the other species is 



