74 CHARLES DARWIN. 



attacks of the feline carnivora " ; the passenger pigeon 

 with less powerful wings could not always procure 

 sufficient food. Hence species thus modified would 

 gradually diminish ; but, on the other hand, if 

 modified in the direction of increased powers, would 

 become more numerous. Varieties will fall under 

 these two classes — those which will never rival, and 

 those which will eventually outnumber, the parent 

 species. If, then, some alteration in conditions 

 occurred making existence more difficult to a certain 

 species, first the less favourable variety would suffer 

 and become extinct, then the parent species, while 

 the superior variety would alone remain, " and on a ; 

 return to favourable circumstances would rapidly 

 increase in numbers and occupy the place of the 

 extinct species and variety." 



The superior variety would thus replace the 

 species, to which it could not return, for the latter 

 could never compete with the former. Hence a 

 tendency to revert would be checked. But the 

 superior variety, when established, would in time give 

 rise to new varieties, some of which would become 

 predominant. Hence 'progression and continued 

 divergence would follow, but not invariably, for the] 

 criteria of success or failure would vary, and would] 

 sometimes render a race which was under other] 

 conditions the most favoured now the least so.j 

 Variations without any effect on the life-preservingj 

 powers might also occur. But it is contended that] 

 certain varieties must, on the average, tend to persist 



