344 Darwin, and after Darwin. 
concerned, the only wonder would be if two different 
lines of descent Aad 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 évans- 
Jormed into another single rational species}. 
In view of such considerations, another and exactly 
1 Principles of Geology, vol. ii. p. 487 (11th ed.). 
