1886. ] The Limits of Organic Evolution. 421 
The second question, concerning the probability of the various 
groups becoming extinct after reaching their culmination, is not 
so easy to answer. It is certainly possible to conceive of them 
as remaining stationary at their culmination, neither developing 
further nor becoming extinct. Undoubtedly the history of the 
past shows that after any group reaches an extreme of specializa- 
l tion it does not remain stationary, but begins to decrease in num- 
; bers, finally to disappear. But the number of groups which have 
; thus become extinct is not very great, and it is a question whether 
it is justifiable to claim that they really represent a general ten- 
i dency. It is certain that disturbing causes which have acted in 
! the past to produce extinction will grow less and less in the 
future. We can see that extinction in the past has been due to: 
the inability of these extreme forms to adapt themselves to new 
circumstances with sufficient readiness. Of course when all of 
our present groups shall have developed themselves to extremes, 
they too will be unable to adapt themselves to new conditions, 
and would doubtless become extinct if they were called upon to 
meet adverse circumstances. But it is an acknowledged fact that 
physical changes are much less rapid now than they were for- 
merly, and that they are constantly diminishing. If this is the 
case the developed extremes of the future will not be called upon 
to meet such changes in condition as those which have induced 
extinction in the past; and they may even then be able to un- 
dergo such slight modifications as will enable them to meet the 
slight changes in condition. Moreover, in the past extinctions: 
have very generally occurred because animals have been unable 
to contend with the new and more vigorous forms which were 
capable of a more rapid modification than the older ones. But 
as we have seen, the number of possible new forms is constantly 
decreasing, and the time must come when it is no longer possible 
s new forms to arise to crowd the older ones out of existence. 
With almost stationary physical conditions, and with no new 
rivals, it may be that the animal kingdom is approaching a con- 
; dition when, for reasons which we have seen, it cannot advance, 
and when there will be nothing to cause extinction, and it will © 
. therefore remain stationary. ae 
4 There is one new condition, however, which is to have a pr 
~ digious influence upon the evolution of the future. The influ- 
b aea ie ee 
_ Ace of man on the animal kingdom cannot be computed, but it = 
