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.

As this conversion of a development passing through different young
states into a more direct one, is not the consequence of a mysterious
inherent impulse, but dependent upon advances accidentally presenting
themselves, it may take place in the most nearly allied animals in the
most various ways, and require very different periods of time for its
completion. There is one thing, however, that must not be overlooked
here. The historical development of a species can hardly ever have taken
place in a continuously uniform flow; periods of rest will have
alternated with periods of rapid progress. But forms, which in periods
of rapid progress were severed from others after a short duration, must
have impressed themselves less deeply upon the developmental history of
their descendants, than those which repeated themselves unchanged,
through a long series of successive generations in periods of rest.
These more fixed forms, less inclined to variation, will present a more
tenacious resistance in the transition to direct development, and will
maintain themselves in a more uniform manner and to the last, however
different may be the course of this process in other respects.

In general, as already stated, it will be advantageous to the young to
commence the struggle for existence in the form of their parents and
furnished with all their advantages--in general, but not without
exceptions. It is perfectly clear that a brood capable of locomotion is
almost indispensable to attached animals, and that the larvae of
sluggish Mollusca, or of worms burrowing in the ground, etc., by
swarming briskly through the sea perform essential services by
dispersing the species over wider spaces. In other cases a metamorphosis