344 Darwin, and after Darwin. 



concerned, the only wonder would be if two different 

 lines of descent had happened to present two in- 

 dependent and yet perfectly parallel lines of history. 



These general considerations would apply equally 

 to the great majority of other cases where some types 

 have made great advances upon others, notwithstand- 

 ing that we can see no reason why the latter should 

 not in this respect have imitated the former. But 

 there is yet a further consideration which must be 

 taken into account. The struggle for existence is 

 always most keen between closely allied species, be- 

 cause, from the similarity of their forms, habits, needs, 

 &c., they are in closest competition. Therefore it often 

 happens that the mere fact of one species having made 

 an advance upon others of itself precludes the others 

 from making any similar advance: the field, so to 

 speak, has already been occupied as regards that 

 particular improvement, and where the struggle for 

 existence is concerned possession is emphatically nine 

 points of the law. For example, to return to the 

 case of apes becoming men, the fact of one rational 

 species having been already evolved (even if the 

 rational faculty were at first but dimly nascent) must 

 make an enormous change in the conditions as 

 regards the possibility of any other such species being 

 subsequently evolved unless, of course, it be by 

 way of descent from the rational one. Or, as Sir 

 Charles Lyell has well put it, two rational species can 

 never coexist on the globe, although the descendants 

 of one rational species may in time become trans- 

 formed into another single rational species l . 



In view of such considerations, another and exactly 



1 Principles of Geology, vol. ii. p. 487 (nth ed.). 



