252 Retrograde Varieties 



group the cross is not at all disting^iished from 

 a normal fertilization, and ordinarily these 

 characters are simply left out of consideration. 

 But it should never be forgotten that they con- 

 stitute the enormous majority, amounting to 

 hundreds and thousands, whereas the differen- 

 tiating marks in each case are only one or two 

 or a few at most. The whole discussion is to 

 be limited to these last-named exceptions. We 

 must consider first what would be the nature of 

 a cross when species are symmetrically com- 

 bined, and what must be the case when varieties 

 are subjected to the same treatment. In so do- 

 ing, I intend to limit the discussion to the most 

 typical cases. We may take the crosses between 

 elementary species of the same or of very nar- 

 rowly allied systematic species on the one side, 

 and on the other, limit treatment to the crossing 

 of varieties with the species, from which they 

 are supposed to have sprung by a retrograde 

 modification. Crosses of different varieties of 

 the same species with one another obviously 

 constitute a derivative case, and should only be 

 discussed secondarily. And crosses of varie- 

 ties with positive or degressive characters have 

 as yet so rarely been made that we may well 

 disregard them. 



Elementary species differ from their nearest 

 allies by progressive changes, that is by the ac- 



