292 EVOLUTION [CHAP. IV 



Letter 211 consideration crosses between A s and A F , B s and B F , with 

 their various approaches to sterility, as I believe they will 

 not affect the final result, although they will greatly complicate 

 the problem. 



14. In the first generation there will result: ist, The 

 pure progeny of A s and of B s ; 2nd, The pure progeny of 

 A F and of B F ; and 3rd, The hybrid progeny of A F , B F . 



15. Supposing that, in ordinary years, the increased 

 constitutional vigour of the hybrids exactly counterbalances 

 their imperfect adaptations to conditions, there will be in the 

 second generation, besides these three classes, hybrids of the 

 second degree between the first hybrids and A F and B F re- 

 spectively. In succeeding generations there will be hybrids of 

 all degrees, varying between the first hybrids and the almost 

 pure types of A F and B F . 



1 6. Now, if at first the number of individuals of A s , B s , 

 A F and B F were equal, and year after year the total number 

 continues stationary, I think it can be proved that, while half 

 will be the pure progeny of A s and B s , the other half will 

 become more and more hybridised, until the whole will be 

 hybrids of various degrees. 



17. Now, this hybrid and somewhat intermediate race 

 cannot be so well adapted to the conditions of life as the two 

 pure species, which have been formed by the minute adapta- 

 tion to conditions through Natural Selection ; therefore, in a 

 severe struggle for existence, the hybrids must succumb, 

 especially as, by hypothesis, their fertility would not be so 

 great as that of the two pure species. 



1 8. If we were to take into consideration the unions of 

 A s with A F and B s with B F , the results would become very 

 complicated, but it must still lead to there being a number of 

 pure forms entirely derived from A s and B s , and of hybrid 

 forms mainly derived from A F and B F ; and the result of the 

 struggle of these two sets of individuals cannot be doubtful. 



19. If these arguments are sound, it follows that sterility 

 may be accumulated and increased, and finally made com- 

 plete by Natural Selection, whether the sterile varieties 

 originate together in a definite portion of the area occupied 

 by the two species, or occur scattered over the whole area. 1 



1 The first part of this discussion should be considered alone, as it is 

 both more simple and more important. I now believe that the utility, and 



