252 Retrograde Varieties 



group the cross is not at all distinguished 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 sexually combined, and 

 7 what must be the case when varieties are sub- 

 jected to the same treatment. In so doing, I in- 

 tend to limit the discussion to the most typical 

 cases. We may take the crosses between ele- 

 mentary 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 ae- 



