No. 610] MUTATION THEORY AND SPECIES-CONCEPT 579 



must be added from experimental work the conceptions 

 of mutants, Mendelian units, phaenotypes and genotypes, 

 and pure lines differing only in the position of their modal 

 condition and requiring statistical analysis for their 

 demonstration. 



With such an array of apparently (though not really) 

 conflicting concepts before him, it is small wonder that 

 the systematist is inclined to cast them all aside and de- 

 scribe his species according to his own ideas of what they 

 are and how they have originated. Nevertheless, for him 

 as for the experimentalist the question What is a species? 

 is more or less colored, if not determined by the question 

 What is its origin? How did it appear? It might be said 

 that the systematist should pay no attention to origins at 

 all, merely describing what he sees. Some systematists 

 doubtless adopt this plan. But obviously, for an under- 

 standing of the characters and relationships of species 

 all possible facts and conceptions bearing on their origin 

 should be considered, and in this way systematics may 

 ultimately hope to become something more than a purely 

 descriptive science. 



If we examine the ideas which form the background of 

 thought of the systematist in his work of constructing 

 species, we find almost invariably that they are based en- 

 tirely upon the Darwinian conception of natural selection 

 by the gradual accumulation of" slight individual differ- 

 ences. We are then concerned to ask, Is the systematist 

 justified in assuming that all specific and varietal differ- 

 ences have originated in one way? I can find no reason 

 in logic or philosophy why this should be the case; and 

 for ourselves, we believe there is no single method of 

 species formation, but we think the conclusion will ulti- 

 mately be reached that the methods of species formation 

 are multiform, though certain of them are doubtless more 

 widespread and important than others. Among the more 

 important factors in speciation which we wish to consider 

 here should therefore be mentioned (1) local adaptation 

 of races through natural selection or by direct response 



