cells or
aggregates of cells had been slightly modified by the action of some
disturbing cause, the cast-off gemmules or atoms of the cell-contents could
hardly fail to be similarly affected, and consequently would reproduce the
same modification. This process might be repeated until the structure of
the part at this particular stage of development became greatly changed,
but this would not necessarily affect other parts whether previously or
subsequently developed. In this manner we can understand the remarkable
independence of structure in the successive metamorphoses, and especially
in the successive metageneses of many animals.

The term growth ought strictly to be confined to mere increase of size, and
development to change of structure.[916] Now, a child is said to grow into
a man, and a foal into a horse, but, as in these cases there is much change
of structure, the process properly belongs to the order of development. We
have indirect evidence of this in many variations and diseases supervening
during so-called growth at a particular period, and being inherited at a
corresponding period. In the case, however, of diseases which supervene
during old age, subsequently to the ordinary period of procreation, and
which nevertheless are sometimes inherited, as occurs with brain and heart
complaints, we {390} must suppose that the organs were in fact affected at
an earlier age and threw off at this period affected gemmules; but that the
affection became visible or injurious only after the prolonged growth of
the part in the strict sense of the word. In all the changes of structure
which regularly supervene during old age, we see the effects of
deteriorated growth, and not of true development.

In the so-called process of _alternate generation_ many individuals are
generated asexually during very early or later stages of development. These
individuals may closely resemble the preceding larval form, but generally
are wonderfully dissimilar. To understand this process we must suppose that
at a certain stage of development the gemmules are multiplied at an unusual
rate, and become aggregated by mutual affinity at many centres of
attraction, or buds. These buds, it may be remarked, must include gemmules
not only of all the succeeding but likewise of all the preceding stages of
development; for when mature they have the power of transmitting by sexual
generation gemmules of all the stages, however numerous these may be. It
w