With respect to the final cause of the young in
such groups not passing through any metamorphosis, we can see that this
would follow from the following contingencies: namely, from the young
having to provide at a very early age for their own wants, and from their
following the same habits of life with their parents; for in this case it
would be indispensable for their existence that they should be modified in
the same manner as their parents. Again, with respect to the singular fact
that many terrestrial and fresh-water animals do not undergo any
metamorphosis, while marine members of the same groups pass through various
transformations, Fritz Muller has suggested that the process of slowly
modifying and adapting an animal to live on the land or in fresh water,
instead of in the sea, would be greatly simplified by its not passing
through any larval stage; for it is not probable that places well adapted
for both the larval and mature stages, under such new and greatly changed
habits of life, would commonly be found unoccupied or ill-occupied by other
organisms. In this case the gradual acquirement at an earlier and earlier
age of the adult structure would be favoured by natural selection; and all
traces of former metamorphoses would finally be lost.
If, on the other hand, it profited the young of an animal to follow habits
of life slightly different from those of the parent-form, and consequently
to be constructed on a slightly different plan, or if it profited a larva
already different from its parent to change still further, then, on the
principle of inheritance at corresponding ages, the young or the larvae
might be rendered by natural selection more and more different from their
parents to any conceivable extent.
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