Morphological Criteria of Phyletic Relationships

SEVERAL years ago the author made a taxonomic study of a relatively large genus of snakes. The number of available specimens was small, data on the natural history of the various forms were lacking, and no recourse could be had to paleontological evidence. The attempt to establish the phyletic relationships of the known species was made from morphological and incomplete geographic data alone. This situation was far from unique; most taxonomists at one time or another are faced with the same problem and they formulate certain principles to aid them in their work. But these principles, seldom clearly voiced, are frequently tenuous or are inconsistently applied. This paper is an attempt to enunciate some of these principles and to formulate criteria which may be of value in phyletic studies. The basic assumption upon which all taxonomic practices rest is that organisms are related. This may be called the principle of similarity. It has been stated repeatedly in various ways, each statement qualifying or buttressing certain phases of the principle but in no way altering its basic meaning. The assumption as stated is open to criticism; what is meant by the phrase, organisms? The phrase ordinarily implies that organisms are made of the same substances, have developed under the control of the same genetic factors, and have developed in the same way. But if all of these processes were truly the same the organisms would, in fact, be identical. The very use of the term similar carries with it a connotation of degree. Organisms are if they are formed from substances under the control of modifiers in a way. This may seem a cyclic argument-organisms are if they are similar-but it is more than that. The intended implication is that organisms are in direct proportion to the degree of similarity of all phases of their existence. As there are degrees of similarity, so must there be degrees of relationship. This should be expressed as a percentage of developmental and metabolic factors shared by two organisms; the greater the percentage of common factors, the closer the relationship. The taxonomist is, however, seldom in a position to determine the physiological factors shared by two organisms. He assumes that if two structures or organisms are similar, then the factors controlling the development of each structure or organism must in part be identical, and that the organisms are related. However, it is not always easy to apply this principle of similarity. Often superficially organisms are not as closely related as dissimilar ones. Characters such as color, size, and shape, are usually subject to rigorous selection by the fluctuating environment, with the result that only a limited variety of forms is tolerated in any particular complex of environmental factors. Thus organisms, when exposed to the same environment, may tend to approach each other morphologically. These superficial similarities obscure their true relationship. Internal characters, which are often less variable, are of much greater value than most external ones in establishing relationship, but frequently are less accessible to the taxonomist. Camp (1916) discovered a new salamander in the Yosemite of California. He placed this species, platycephalus, in a Mexican genus,