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A. W. LINDSEY 
be treated as they are at present, and together with groups of 
all other degrees of stability. The transition is so gradual that 
different terms cannot be used to designate conveniently the vari- 
ous conditions, therefore it becomes necessary to arrive at an ac- 
curate idea of the variable application of that now in use, namely, 
species. Since, in a consideration of phylogeny, the present must 
necessarily refer to geologic time, the result may well apply to 
all scientific results of the past, and to the future beyond logical 
prediction. 
In order to be compatible with the ideal species, “species” se- 
lected from the more variable forms should be definitely delimit- 
able groups of individuals, regardless of the amount of variation 
included. This is essentially the idea of Powersh Montgomery 
expresses a very convenient idea of species, although with a very 
different intent, which may be used in this connection. This is 
found in his definition of a species as a “mental section of a line 
of evolution.”^ The opinion has already been expressed that 
these sections may be natural entities, and that in order to be 
properly regarded as species, they must be. In other words, 
in an hypothetical cross section of the phylogeny of all existing 
forms, the species are those elements which are definitely sepa- 
rable. When elements named as species in the past are found 
incapable of separation, they must be regaredd as divisions in- 
ferior to the species of this interpretation, and the species to 
which they belong should be sought in a “section” of greater ex- 
tent. The accurate arrangement of any of these elements in our 
actual classification is, of course, fraught with considerable dif- 
ficulty. Until our knowledge is more nearly complete and per- 
fect we must expect constantly to revise our results; the careful 
and diligent study of an abundance of material — ideal conditions, 
to be sure, — may some day result in a classification of reason- 
able stability. 
It is not the writer’s purpose to present here a discussion of 
the problems of transmutation of species,^ the influence of gen- 
etics on the species concept-’ or the methods of limitation of 
species,^ which are taken up in the papers cited. It is proposed 
to suggest in this concept of species a basis for practical classifi- 
cation. By reducing the application of the term species to groups 
® Montgomery, cp. cit., p. 206. 
^ Powers, op. cit. 
® Davis, Species, Pure and Impure, Science Iv, no. 1414, pp. 107-114, 1922. 
® Montgomery, op. cit. 
