beings produced sexually and
asexually, which is very general. The former usually pass in the course of
their development from a lower to a higher grade, as we see in the
metamorphoses of insects and in the concealed metamorphoses of the
vertebrata; but this passage from a lower to a higher grade cannot be
considered as a necessary accompaniment of sexual reproduction, for hardly
anything of the kind occurs in the development of Aphis amongst insects, or
with certain crustaceans, cephalopods, or with any of the higher vascular
plants. Animals propagated asexually by buds or fission are on the other
hand never known to undergo a retrogressive metamorphosis; that is, they do
not first sink to a lower, before passing on to their higher and final
stage of development. But during the act of asexual production or
subsequently to it, they often advance in organisation, as we see in the
many cases of "alternate generation." In thus speaking of alternate
generation, I follow those naturalists who look at the process as
essentially one of internal budding or of fissiparous generation. Some of
the lower plants, however, such as mosses and certain algae, according to
Dr. L. Radlkofer,[880] when propagated asexually, do undergo a
retrogressive metamorphosis. We can to a certain extent understand, as far
as the final cause is concerned, why beings propagated by buds should so
rarely retrogress during development; for with each organism the structure
acquired at each stage of development must be adapted to its peculiar
habits. Now, with beings produced by gemmation,--and this, differently from
sexual reproduction, may occur at any period of growth,--if there were
places for the support of many individuals at some one stage of
development, the simplest plan would be that they should be multiplied by
gemmation at that stage, and not that they should first retrograde in their
development to an earlier or simpler structure, which might not be fitted
for the surrounding conditions.

{362}

From the several foregoing considerations we may conclude that the
difference between sexual and asexual generation is not nearly so great as
it at first appears; and we have already seen that there is the closest
agreement between gemmation, fissiparous generation, the repair of
injuries, and ordinary growth or development. The capacity of fertilisation
by the male element seems to be the chief distinction between an ovule and
a bud; and this capacity