PRINCIPLES OF CLASSIFICATION. 5 



present data, seems the nearest approximation I can make to the true 

 condition, for I well know that its usefulness will soon be past. 



I believe that it is very important that the International Congress of 

 Zoologists should adopt a set of definitions of species and the less inclusive 

 taxonomic grades in order that some uniformity can be attained in the 

 use of such words as varieties, subspecies, forms, aberrations, mutations, 

 etc. Until that time each writer must define his use of such terms when- 

 ever called upon to use them extensively. Failure to do this is causing a 

 constantly growing confusion. 



The following definitions will show the meaning of the terms as here 

 employed. I am well aware that many of these terms are used differently 

 by some writers, but at present each worker can merely choose the use 

 that seems most satisfactory to him. 



A species consists of individuals which, aside from sex, age, and periodic 

 differences, have a fundamental similarity and which are habitually and 

 successfully interbreeding. They comprise an intergenerating unit. 



A section is a part of a species which is cut off by some barrier from 

 intergeneration. Where the individuals of the section have no apparent 

 difference from the mass of the species the section is ignored. Where 

 the difference is so great that individuals of the two sections are always 

 readily distinguished, a distinctive name becomes desirable and the form 

 is considered another species rather than a section, although experiment- 

 ally the two will interbreed freely. The difference between section and 

 species is one only of degree of difference in the characteristics. The sec- 

 tion may be found to be ontogenetic or phylogenetic in its nature. 



A subspecies is a part of a species which inhabits only a portion of the 

 range of the species and which, while differing in some appreciable respect, 

 intergrades with it in the intermediate region. At any one point through- 

 out the range of the species the quantitative expression of the chief differ- 

 ential of a number of individuals similar in age, sex, and season should 

 give a unimodal polygon of frequency. The term "subspecies" maybe 

 used without reference to the inheritability of the differences. But where 

 these differences are found primarily because they are the result of 

 environment in each generation, the subspecies is called an ontogenetic 

 subspecies. Where it is present primarily because inherited it is called a 

 phylogenetic subspecies. 



If the conditions of a subspecies are fulfilled, except that the individuals 

 in question are scattered geographically and are confined to a particular 

 kind of habitat throughout its range, then we have a habitat-form, 

 which may be ontogenetic or phylogenetic. The usual assumption that all 

 habitat-forms are ontogenetic is, I think, questionable, although they are 

 probably generally so. The distinction between a subspecies and a habitat- 

 form is not a sharp one, for a subspecies is a habitat-form where the 

 habitat involves a very large, continuous area. The habitat-form falls 



