CRITERIA FOR SUBSPECIES, SPECIES AND GENERA, AS DETERMINED BY RESEARCHES ON FISHES

The criteria for the minor taxonomic groups of fishes do not differ essentially from those that are pertinent to the subspecies, species and genera of the tetrapod clrtsses. The increasingly detailed and penetrating researches of recent years keep emphasizing the conclusion that these categories are of essentially the same sorts in all groups of vertebrates. Speciational processes appear to be similar throughout the Vertebrata, in fact throughout most of the organic world. For several reasons, however, we find that fishes are particularly well suited to an analysis of the minor taxonomic categories, and hence to an appreciation of the criteria by which these assemblages can be recognized. In the first place, fishes are rich in differentiae: they fairly bristle with diagnostic external features, and charactera penetrate their whole anatomy (the internal characters may readily be investigated, for the whole bodies-not just skins-are preserved). Most of the distinguishing features are readily subject to precise evaluation, and hence to statistical analysis; meristic differences are common. For such studies it is often poasible to make use of hundreds or even thousands of specimens, for large series of fish are readily collected and preserved. The critical relations between individual and racial modifications are particularly amenable to study in fishes, for the aquatic vertebrates are molded by their environment to a greater degree than are the representatives of the so-called higher groups; the trend of vertebrate evolution has been to free the organism from domination by its physical environment. The fact that many of the stocks or races within a fish species occupy highly diverse ecological situations often makes it possible, without recourse to experiment, to analyze the individual modifications and the genetic respow, to the environment. Finally, the genetic basis of characters and the genetic interrelationships of forms can be determined more readily by experiment in the Pisces than in most other groups, because many fishes may readily be propagated, and because of the frequent occurrence, in certain fishes, not only of subspecific intergradation,