Chap. IX. 
GEOLOGICAL KECOED. 
281 
determined from a mere comparison of their structure 
with that of the rock-pigeon^ whether they had descended 
from this species or from some other allied species, such 
as C. oenas. 
So with natural species, if we look to forms very 
distinct, for instance to the horse and tapir, we have 
no reason to suppose that links ever existed directly 
intermediate between them, but between each and an 
unknown common parent. The common parent will 
have had in its whole organisation much general resem¬ 
blance to the tapir and to the horse; but in some points 
of structure may have differed considerably from both, 
even perhaps more than they differ from each other. 
Hence in all such cases, we should be unable to recog¬ 
nise the parent-form of any two or more species, even 
if we closely compared the structure of the parent with 
that of its modified descendants, unless at the same 
time we had a nearly perfect chain of the intermediate 
links. 
It is just possible by my theory, that one of two living 
forms might have descended from the other; for in¬ 
stance, a horse from a tapir; and in this case direct 
intermediate links will have existed between them. 
But such a case would imply that one form had re¬ 
mained for a very long period unaltered, whilst its 
descendants had undergone a vast amount of change; 
and the principle of competition between organism and 
organism, between child and parent, will render this a 
very rare event; for in all cases the new and improved 
forms of life tend to supplant^ the old and unimproved 
forms. 
By the theory of natural selection all living species 
have been connected with the parent-species of each 
genus, by differences not greater than we see be¬ 
tween the varieties of the same species at the present 
