108 
NATUKAL SELECTION. 
Chap. IV. 
vented, so that new places in the polity of each island 
will have to be filled up by modifications of the old in¬ 
habitants ; and time will be allowed for the varieties in 
each to become well modified and perfected. When, by 
renewed elevation, the islands shall be re-converted into 
a continental area, there will again be severe competi¬ 
tion : the most favoured or improved varieties will be 
enabled to spread: there will be much extinction of 
the less improved forms, and the relative proportional 
numbers of the various inhabitants of the renewed con¬ 
tinent will again be changed; and again there will be 
a fair field for natural selection to improve still further 
the inhabitants, and thus produce new species. 
That natural selection will always act with extreme 
slowness, I fully admit. Its action depends on there 
being places in the polity of nature, which can be better 
occupied by some of the inhabitants of the country 
undergoing modification of some kind. The existence 
of such places will often depend on physical changes, 
which are generally very slow, and on the immigration 
of better adapted forms having been checked. But the 
action of natural selection will probably still oftener de¬ 
pend on some of the inhabitants becoming slowly modi¬ 
fied ; the mutual relations of many of the other inha¬ 
bitants being thus disturbed. Nothing can be effected, 
unless favourable variations occur, and variation itself is 
apparently always a very slow process. The process will 
often be greatly retarded by free intercrossing. Many 
will exclaim that these several causes are amply suffi¬ 
cient wholly to stop the action of natural selection. I do 
not believe so. On the other hand, I do believe that 
natural selection always acts very slowly, often only 
at long intervals of time, and generally on only a very 
few of the inhabitants of the same region at the same 
time. I further believe, that this very slow, intermit- 
