INTRODUCTION. 11 



without concern as to their origin and relationship, while the true method of taxonomy 

 is to deal with the tree as a whole, and to regard the units in it as so much correlated 

 evidence of evolution. 



The evolutionary definition. — To the student of evolution, all these proposed tests of 

 species are gratuitous, arising from an incomplete view of the field and suffering from 

 the bias of the specialist. While gametic purity, constancy, and recognizability all have 

 their meaning, this is as yet too little understood to make them little more than inter- 

 esting working hypotheses. There is no warrant in our present knowledge of the course 

 of evolution for assuming that they have any more connection with the species than 

 with the most recent ecad or mutant. The evolutionary view of the species is that it 

 is a definite phylogenetic stock, sprung from and related to similar stocks, and itself 

 undergoing modification into a number of variads. As they have recently come from 

 the same stock, these variads are more nearly related to each other than they are to those 

 of any other species, and they represent a definite phylogenetic unit, the species, at the 

 same time that they mark its further differentiation. The only definite measure of the 

 progress of evolution is found in the degree of morphological difference, and species 

 necessarily share this morphological basis with other units. To ask that all species show 

 the same degree of morphological difference is to misunderstand the nature of evolu- 

 tion, but it is possible to demand that the great majority of them show a definite dif- 

 ference in the proper position in the sequence of units. In short, a species must not 

 only show adequate morphological differentiation, but this must bear a definite relation 

 to that of the genus on the one hand and of the variads on the other. It seems a truism 

 to point out that in an evolutionary taxonomy each unit must be determined as much 

 or more by its relation to the unit that precedes and the one that follows as by its purely 

 structural characters. 



The inadequacy of a strict morphological basis for species is due chiefly to the para- 

 mount role of divergence. If two species have been differentiated from an original stock 

 by the impact of reciprocal factors, such as greater wetness on the one hand and dryness 

 on the other, this very divergence will give opportunity for convergence when they 

 invade reverse habitats. This convergence may sometimes become practical identity 

 (Clements, 1904), but as a rule ancestral or related characters will furnish the clue to 

 descent. Thus, while most species will show distinct gaps, it must be admitted that the 

 gap may be quite or completely closed in some instances, in which evolutionary analysis 

 is alone of avail. In this task ecology is often of great assistance in relating response to 

 habitat and in connecting differentiating habitats with the habitat of the ancestral 

 mass. Thus, while evolutionary taxonomy does not pretend to offer a definition of the 

 species, it does provide the method by which species can be recognized and by which 

 they can be related to each other as well as to the major and minor units. The conse- 

 quences of various attitudes toward the species are discussed in the two following 

 sections, and the details of the evolutionary method are dealt with in the last section, 

 as well as under the later caption "Methods and results in evolutionary taxonomy." 



The segregation of species. — The method of segregation that has characterized descrip- 

 tive botany in America during the past quarter of a century is a necessary consequence of 

 the idea that everything distinguishable is a species. Applied in the field, this definition 

 would have worked little damage, as the abundance of material would quickly have set 

 a limit to its operation. In the herbarium, however, the small number of individuals 

 represented and the frequently incomplete nature of the specimens magnified small 

 differences and gave the impression of gaps where none actually existed. In spite of 

 the assumption of many descriptive botanists that much of their work is done in the 

 field, this rarely amounts to little more than collecting a few individuals of the out- 

 standing forms, and the describing is wholly a matter of the herbarium. Collecting for 



