The historical record preserved in developmental history is gradually
EFFACED as the development strikes into a constantly straighter course
from the egg to the perfect animal, and it is frequently SOPHISTICATED
by the struggle for existence which the free-living larvae have to
undergo.
Thus as the law of inheritance is by no means strict, as it gives room
for individual variations with regard to the form of the parents, this
is also the case with the succession in time of the developmental
processes. Every father of a family who has taken notice of such
matters, is well aware that even in children of the same parents, the
teeth, for example, are not cut or changed, either at the same age, or
in the same order. Now in general it will be useful to an animal to
obtain as early as possible those advantages by which it sustains itself
in the struggle for existence. A precocious appearance of peculiarities
originally acquired at a later period will generally be advantageous,
and their retarded appearance disadvantageous; the former, when it
appears accidentally, will be preserved by natural selection. It is the
same with every change which gives to the larval stages, rendered
multifarious by crossed and oblique characters, a more straightforward
direction, simplifies and abridges the process of development, and
forces it back to an earlier period of life, and finally into the life
of the egg.
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