320 
GEOLOGICAL SUCCESSION. 
Chap. X. 
man’s agency. I may repeat what I published in 1845, 
namely, that to admit that species generally become 
rare before they become extinct—to feel no surprise at 
the rarity of a species, and yet to marvel greatly when 
it ceases to exist, is much the same as to admit that 
sickness in the individual is the forerunner of death— 
to feel no surprise at sickness, but when the sick man 
dies, to wonder and to suspect that he died by some 
unknown deed of violence. 
The theory of natural selection is grounded on the 
belief that each new variety, and ultimately each new 
species, is produced and maintained by having some 
advantage over those with which it comes into compe¬ 
tition ; and the consequent extinction of less-favoured 
forms almost inevitably follows. It is the same with 
our domestic productions : when a new and slightly 
improved variety has been raised, it at first supplants 
the less improved varieties in the same neighbourhood; 
when much improved it is transported far and near, 
like our short-horn cattle, and takes the place of other 
breeds in other countries. Thus the appearance of new 
forms and the disappearance of old forms, both natural 
and artificial, are bound together. In certain flourishing 
groups, the number of new specific forms which have 
been produced within a given time is probably greater 
than that of the old specific forms which have been 
exterminated; but we know that the number of species 
has not gone on indefinitely increasing, at least during 
the later geological periods, so that looking to later 
times we may believe that the production of new forms 
has caused the extinction of about the same number of 
old forms. 
The competition will generally be most severe, as 
formerly explained and illustrated by examples, between 
the forms which are most like each other in all respects. 
