s, except the mutual struggle between the different individuals
and classes; but from the strong and general hereditary tendency we
might expect to find some tendency to progressive complication in the
successive production of new organic forms.

{479} Compare _Origin_, Ed. i. p. 419, vi. p. 575.

{480} <Note in original.> Scarcely possible to distinguish between
non-development and retrograde development.

_Modification by selection of the forms of immature animals._

I have above remarked that the feline{481} form is quite of secondary
importance to the embryo and to the kitten. Of course, during any great
and prolonged change of structure in the mature animal, it might, and
often would be, indispensable that the form of the embryo should be
changed; and this could be effected, owing to the hereditary tendency at
corresponding ages, by selection, equally well as in mature age: thus if
the embryo tended to become, or to remain, either over its whole body or
in certain parts, too bulky, the female parent would die or suffer more
during parturition; and as in the case of the calves with large hinder
quarters{482}, the peculiarity must be either eliminated or the species
become extinct. Where an embryonic form has to seek its own food, its
structure and adaptation is just as important to the species as that of
the full-grown animal; and as we have seen that a peculiarity appearing
in a caterpillar (or in a child, as shown by the hereditariness of
peculiarities in the milk-teeth) reappears in its offspring, so we can
at once see that our common principle of the selection of slight
accidental variations would modify and adapt a caterpillar to a new or
changing condition, precisely as in the full-grown butterfly. Hence
probably it is that caterpillars of different species of the Lepidoptera
differ more than those embryos, at a corresponding early period of life,
do which remain inactive in the womb of their parents. The parent during
successive ages continuing to be adapted by selection for some one
object, and the larva for quite another one, we need not wonder at the
difference becoming wonderfully great between them; even as great as
that between the fixed rock-barnacle and its free, crab-like offspring,
which is furnished with eyes and well-articulated, locomotive
limbs{483}.

{481} See p. 42, where the same illustration is used.

{482} _Var. under Dom._, Ed. ii. vol. I. p. 452.

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