This circumstance seems to be explicable in
two ways. Either species without a metamorphosis migrated especially
into the fresh waters, or the metamorphosis was more rapidly got rid of
in the emigrants than in their fellows remaining in the sea.
Animals without a metamorphosis would naturally transfer themselves more
easily to a new residence, as they had only themselves and not at the
same time multifarious young forms to adapt to the new conditions. But
in the case of animals with a metamorphosis, the mortality among the
larvae, always considerable, must have become still greater under new
than under accustomed conditions, every step towards the simplification
of the process of development must therefore have given them a still
greater preponderance over their fellows, and the effacing of the
metamorphosis must have gone on more rapidly. What has taken place in
each individual case, whether the species has immigrated after it had
lost the metamorphosis, or lost the metamorphosis after its immigration,
will not always be easy to decide. When there are marine allies without,
or with only a slight metamorphosis, like the Lobster as the cousin of
the Cray-fish, we may take up the former supposition; when allies with a
metamorphosis still live upon the land or in fresh water, as in the case
of Gecarcinus, we may adopt the latter.
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