362 COOK 



merely abstract conception, that species do not exist, and can 

 not be defined. 1 



Those who have not persevered beyond this stage of skepticism 

 and satisfied themselves of the existence of species in nature, 

 can have little use for an interpretation based on the recognition 

 of species as definite entities, consisting not merely of aggre- 

 gates of individual organisms, but also of fabrics of interwoven 

 lines of descent. 



The difficulty in defining species is the lack of clear percep- 

 tions, not only of the nature and constitution of species, but also 

 of the fact that several diverse types of phenomena are being 

 covered by the word. Under such circumstances a general 

 definition of species, however framed, could afford only a ficti- 

 tious unification of expression, the ideas and implications cov- 

 ered by the term remaining essentially diverse and often quite 

 contradictory. This confusion affords, however, no justifica- 

 tion of a failure to use the term in one or another of the explicit 

 senses of which it is capable, nor of a refusal to define the usage 

 of the term in any particular connection. 



The difficulty of defining the term species has arisen mostly 

 from the fact that the phenomenon is a physiological one, 

 whereas the general supposition has been that it is morpho- 

 logical. The idea that species are " founded on identity of 

 form and structure," as the dictionaries say, is still widely 

 prevalent, and is one of the tenets of evolutionary belief upon 

 which Professor De Vries especially insists. 



The impracticability of a morphological definition of species 

 arises from the fact that it is impossible to set definite limits to 

 the extent of the variability or diversity which is to be permitted 

 in the species. Identity of form and structure makes an excellent 

 definition ; the objection to it is that no such species seem to 

 exist in nature, or as Professor De Vries says, " * * * purely 

 uniform species seem to be relatively rare." 2 In some groups 



1 Thus a recent defender of the mutation theory of De Vries has declared : " If 

 it is really true that De Vries does not know what constitutes a species, then, 

 indeed, we find our faith in his work thereby increased. Who, indeed, except 

 the makers of dictionaries, does ' know what constitutes a species ' ? " 



This method of reasoning was very popular in mediaeval times and was then, 

 reduced to the neatly pious formula: " Credo quia absurdum." 



2 De Vries, H., 1905. Species and Varieties, 64. 



