HYBRIDISM 27 



have been rendered utterly impotent on a second form, 

 while at the same time the male element of this second 

 form is enabled freely to fertilize the first form; for this 

 peculiar state of the reproductive system could hardly 

 have been advantageous to either species. 



In considering the probability of natural selection 

 having come into action, in rendering species mutually 

 sterile, the greatest difficulty will be found to lie in the 

 existence of many graduated steps from slightly lessened 

 fertility to absolute sterility. It may be admitted that it 

 would profit an incipient species if it were rendered in 

 some slight degree sterile when crossed with its parent 

 form or with some other variety; for thus fewer bastard- 

 ized and deteriorated offspring would be produced to 

 commingle their blood with the new species in process 

 of formation. But he who will take the trouble to re- 

 flect on the steps by which this first degree of sterility 

 could be increased through natural selection to that high 

 degree which is common with so many species, and which 

 is universal with species which have been differentiated 

 to a generic or family rank, will find the subject extraor- 

 dinarily complex. After mature reflection it seems to me 

 that this could not have been effected through natural 

 selection. Take the case of any two species which, when 

 crossed, produced few and sterile offspring; now, what is 

 there which could favor the survival of those individuals 

 which happened to be endowed in a slightly higher de- 

 gree with mutual infertility, and which thus approached 

 by one small step toward absolute sterility ? Yet an ad- 

 vance of this kind, if the theory of natural selection be 

 brought to bear, must have incessantly occurred with 

 many species, for a multitude are mutually quite barren. 



