1884.] Growth, its Conditions and Variations. : 1215 
was less one of personal battle than of success in obtaining food 
from a common source, then it does not appear at first sight evi- 
dent why the different dominant types should not have over- 
lapped, the great reptiles, for instance, continuing into the Eocene 
instead of completely abandoning the field to the mammals. 
There is a probable cause of this, to which no attention has 
hitherto been called, yet which may have been amply sufficient 
in all such cases. Thus the great carnivorous oceanic reptiles 
pursued prey which they must have found continually greater 
difficulty in capturing. The easily taken forms either vanished 
or changed in organization and habits, growing smaller, swifter, 
and more wary. They also became continually better adapted to 
the modes of attack of their foes, so that the difficulty of the 
latter in obtaining sufficient food for their needs must have steadily 
increased, and a retrograde movement in size and numbers have 
arisen. Under such circumstances a new-comer, which had at- 
tained size and strength in another region, must have possessed a 
peculiar advantage. The swimming food had learned the art of 
self defence or escape from its older foes, but lay helpless before 
this new foe, to whose mode of attack it was not accustomed. 
Such a new-comer would therefore be able to make fierce 
havoc in the ranks of the food animals, and rapidly cut down the 
harvest. It would tend to a rapid increase in bulk and strength, 
While its older competitors must shrink in size or perish, starved 
out of existence. Even when otherwise equal in organization the 
New-comer would have an immense advantage in the lack of 
adaptation of the food to its weapons and methods of assault. It 
'S perhaps largely due to this cause that sudden successions in 
reptilian forms, and in the dominant forms of other types, t 
place, New forms came from distant regions and robbed the 
indigenous forms of the bulk of the food. But when to this 
advantage was added that of a superior organization, as of reptiles 
over batrachians and of mammals over reptiles, the earlier asus 
Would be supplanted yet more rapidly and completely , = 
agg in the dominant forms of great suddenness might take 
sudden replacements 
ological times, may 
d replacement of 
which a dom- 
To this cause of the numerous apparently 
a aoe forms by newer, unrelated forms in ge i 
_ “added another, which refers to a similar rapid 
telated forms. The case so far described is that in 
