580 Species According to the Theory of Mutation. 



variety ;^ or, as we have already expressed it in the first 

 part of this volume (pp. 64, 71, etc.), the origin of new 

 characters leads to specific differentiation, whereas the 

 true derivative varieties arise by so-called retrogressive 

 and degressive mutations without the formation of new 

 characters. In my opinion this is, at any rate, the sim- 

 plest view of the matter. 



This, however, is purely theoretical, for in practice 

 our definition can, at present, only rarely be applied. Here 

 however the principles of hybridization come to our aid ; 

 for, expressed in hybridological terminology, our gen- 

 eralization runs : Forms, all of the characters of which 

 follow Mendel's laws in crosses, are to be regarded as 

 varieties of the same species. This form of our generali- 

 zation obviously admits of an immediate application in 

 every case where the material can be dealt with experi- 

 mentally. 



Obviously this generalization is at present too sweep- 

 ing; nevertheless the best investigators^ have regarded 

 the study of hybridization as an empirical foundation on 

 which this distinction may be based. Moreover the diffi- 

 culties are not really so great as they seem to be at first 

 sight ; for as soon as the body of evidence will have at- 

 tained a certain extent, definite laws will be detected 

 which will fit the majority of cases by judging from anal- 

 ogy. 



The species, however, which I am here distinguishing 

 from varieties are the smaller or elementary species ; the 

 delimitation of the larger or collective species is, from the 

 very nature of the case, a question not for the experi- 



^ It should not be forgotten that varieties have been called In- 

 cipient species and that from seed they are just as constant as species. 



^See Nagelt, loc. cit., p. 396; Focke. loc. cit., pp. 488, 502; Nau- 

 DiN, loc. cit., p. 164 ; Abbado^ loc. cit., p. 9, etc. 



