I turn to the more congenial task of considering the developmental
history of the Crustacea from the point of view of the Darwinian theory.

Darwin himself, in the thirteenth chapter of his book, has already
discussed the conclusions derived from his hypotheses in the domain of
developmental history. For a more detailed application of them, however,
it is necessary in the first place to trace these general conclusions a
little further than he has there done.

The changes by which young animals depart from their parents, and the
gradual accumulation of which causes the production of new species,
genera, and families, may occur at an earlier or later period of
life,--in the young state, or at the period of sexual maturity. For the
latter is by no means always, as in the Insecta, a period of repose;
most other animals even then continue to grow and to undergo changes.
(See above, the remarks on the males of the Amphipoda.) Some variations,
indeed, from their very nature, can only occur when the young animal has
attained the adult stage of development. Thus the Sea Caterpillars
(Polynoe) at first possess only a few body-segments, which, during
development, gradually increase to a number which is different in
different species, but constant in the same species; now before a young
animal could exceed the number of segments of its parents, it must of
course have attained that number. We may assume a similar supplementary
progress wherever the deviation of the descendants consists in an
addition of new segments and limbs.

Descendants therefore reach a new goal, either by deviating sooner or
later whilst still on the way towards the form of their parents, or by
passing along this course without deviation, but then, instead of
standing still, advance still farther.