INTRODUCTION. 9 



is plainly an artificial one. Since there is no intent to minimize analysis when it is in 

 agreement with evolution, it would seem desirable to make rather more of sections as 

 records of recent lines of specialization within the generic stock. This can readily be 

 done by making them more prominent in the treatment of genera, and in special cases 

 by the use of the sectional name or initial in a trinomial. With respect to practica- 

 bility, the use of genus and section alone makes possible the correlations that are essential 

 to a working knowledge of taxonomy. This likewise insures the smallest possible number 

 of genera, which is also a cardinal point in usability. Moreover, this method permits 

 the accumulation of a permanent body of knowledge, both for the subject and the 

 individual, as the constitution of each genus is no longer subject to the personal judg- 

 ment of every specialist. This is especially significant, in view of the fact that the raising 

 of actual sections to genera does not represent new knowledge, but rather an ignoring 

 of the essential facts of relationship. When the latter are given their full value, there 

 will be no question as to the usability, convenience, and attractiveness of taxonomy 

 for all those who wish to make use of it. 



THE SPECIES CONCEPT. 



Nature of the species. — As with the genus, there has been much divergence of views 

 as to whether the species is merely a concept or an actual entity. Apparently, the 

 general opinion is to the effect that the species is a concept, but this seems to deal with 

 the term and its application rather than with the unit itself. Once granted that the 

 term might have been applied to any other unit, or an altogether different term employed, 

 it seems evident that the unit itself is as definite an entity as a plant community. 

 In fact, with many dominants and subdominants, the species and the community are 

 coextensive, and hence equally definite. Like the genus, the species represents a certain 

 portion of the line of evolution, and it lacks definiteness only where evolution has 

 progressed uniformly, without sharp divergences or breaks. Practically all species are 

 still susceptible of modification, and many of them exhibit it most actively. If only 

 the end results of this process are known, as in the case of herbarium studies, they appear 

 to be distinct units, often with little or no evidence of their common origin. As a con- 

 sequence, units of every possible degree of differentiation and value are masquerading 

 as species in existing taxonomy. As long as species are made in the herbarium instead 

 of the field, this condition must continue, at least in some degree, but it will disappear 

 rapidly before statistical and experimental studies in the natural habitats. Such studies 

 have already reached the point where it is possible to relate a number of recent segre- 

 gates to their proper specific stock, and they indicate that this will be the regular out- 

 come of such methods. 



Definition of species. — Many attempts have been made to provide a working definition 

 of the term species, but these have been successful only in leading to widely divergent 

 usages. The old definition based upon sterility has long been discarded, but its place 

 has been taken in some measure by the definition of the geneticists, which is based 

 upon gametic purity. This basis has actually been proposed by Lotsy (1916), and it 

 does have the advantage of being more objective than any other. It is a concept of 

 use to geneticists alone, however, since gametic purity can only be determined by genetic 

 analysis, and this is applicable so far to relatively few cases. Plants entirely identical 

 in all external characters may exhibit a different gametic composition, as indicated by 

 their behavior when hybridized, and would accordingly constitute different species. 

 The classical example of this are the two strains of white sweet peas described by Bateson 

 (1913), and a similar case is found in the 14 strains of corn, all with white aluerone 

 (Babcock and Clausen, 1918). It is obvious that such a definition of the species is 



