1098 ` Growth, its Conditions and Variations. [Novel | 
requisite level in both of these powers, it is in danger of extinction i 
If it rises above the level some other tribe is in danger of extin — 
tion. This latter is a highly important fact, to which further 
attention will be requisite after some other points have been con- 
sidered. 
We may here speak of the great influence which larval meta- 
morphosis seems to have on the ultimate size of animals. With 
the invertebrates, as a general rule, the young come into the 
world half born. They are thrown upon their own resources in 
a partly developed state, and have a high wall of metamorphosis 
to climb ere they can reach the stage af maturity. During this 
larval period they represent animals of a lower grade of organi- 
zation than the mature form, and more imperfectly adapted to 
food getting, except in those cases in which the mature form isa 
degenerated one, or in those other in which the mature form has 
lost its original adaptation to the food conditions, and thus is 
forced into immediate reproductive activity, as in the case o 
many insects. If we take, for instance, the actively feeding cater- 
pillar and compare it with the butterfly, we find the latter, ot 
leaving the pupa case, to be not more than a tenth of the sized! 
its larva, while the new food stock, to which it has become — 
adapted, is much less abundant and easily obtained than that 
open to the caterpillar. Thus growth force ceases, reproducti : 
activity supervenes, and death of the mature animal quicky 
follows, | 
Vertebrates are tided over these larval stages, which they pas 
either in the egg or in the maternal womb. Th 
e loss of it 
vigor in their case falls upon the mother, not upon the —_ 
The marsupials, in which the young are born imperfectly oo 
oped, are all comparatively small in size. This may ate 
ever, arise from the cause specified, since the young obian | 
from the mother without personal exertion until they have 
through the larval stage. As a rule, as pointed out by ae 
Spencer, the initial size of the animal exerts a vigorous i 
upon its ultimate size. If it begins its individual life wih 
bulk and great food-taking and consuming powers, its ed 
will be proportionate. He instances the diversity in si° off 
and oxen feeding in the same pastures. : . 
ee Intelligent selection, as exercised by man upon dan “ 
: _ mals, is capable of producing great variations in siae 
A 
ca 
