584 Species According to the Theory of Mutation. 



ous forms or elementary species and derived forms or 

 trne varieties. 



As we concluded in § 7, Part I, p. 65, the origin of 

 elementary species is due to the formation of new ele- 

 mentary characters, i. e., to their actual numerical in- 

 crease. True varieties differ from the species to which 

 they belong by the latency of certain characters, which 

 may either be active as in the type of the species, or latent 

 as in the variety, or which may occur in a latent or semi- 

 latent condition in the former, and become active or semi- 

 active in the production of the variety. In other w^ords, 

 we may say that elementary species arise by progressive 

 mutations, but derivative varieties by retrogressive and 

 degressive ones (p. 71). 



If we now compare the principles derived from the 

 study of hybrids with these conclusions, we see that the 

 two main types of hybrids are in essential agreement 

 with these two systematic groups. Mendelian hybrids 

 correspond to retrogressive and degressive specific dif- 

 ferentiation, and consequently to true varieties ; uni- 

 sexual hybridizations correspond to progressive specific 

 differentiation and consequently to elementary species. 



There can hardly be any misconception as to the sig- 

 nification of this important conclusion. But it only indi- 

 cates the principle and not its application to particular 

 cases; and, as a matter of fact, it is only another form 

 of the generalization enunciated above relating to the 

 fertility of crosses and hybrids. For Mendelian crosses 

 have as a rule the same fertility as the pure parent forms, 

 and fertility does not diminish in the subsequent hvbrid 

 generations. In uni-sexual crosses, however, fertility di- 

 minishes and it does so in pronortion as the relation be- 

 tween the two forms crossed becomes more remote. 



