ult, such as
bird's teeth and snake's fore-limbs: the adult stage in which they have
disappeared is not preceded by embryonic stages in which the teeth and
limbs or rudiments of them are present. In fact the evidence indicates
that adult variations of any part are accompanied by precedent
variations in the same direction in the embryo. The evidence seems to
show, not that a stage is added on at the end of the life-history, but
only that some of the stages in the life-history are modified. Indeed,
on the wider view of development taken in this essay, a view which makes
it coincident with life, one would not expect often to find, even if new
stages are added in the course of evolution, that they are added at the
end of the series when the organism has passed through its reproductive
period. It is possible of course that new stages have been intercalated
in the course of the life-history, though it is difficult to see
how this has occurred. It is much more likely, if we may judge from
available evidence, that every stage has had its counterpart in
the ancestral form from which it has been derived by descent with
modification. Just as the adult phase of the living form differs, owing
to evolutionary modification, from the adult phase of the ancestor from
which it has proceeded, so each larval phase will differ for the same
reason from the corresponding larval phase in the life-history of the
ancestor. Inasmuch as the organism is variable at every stage of its
independent existence and is exposed to the action of natural selection
there is no reason why it should escape modification at any stage.

If there is any truth in these considerations it would seem to follow
that at the dawn of life the life-cycle must have been, either in posse
or in esse, at least as long as it is at the present time, and that
the peculiarity of passing through a series of stages in which new
characters are successively evolved is a primordial quality of living
matter.

Before leaving this part of the subject, it is necessary to touch upon
another aspect of it. What are these variations in structure which
succeed one another in the life-history of an organism? I am conscious
that I am here on the threshold of a chamber which contains the clue to
some of our difficulties, and that I cannot enter it. Looked at from
one point of view they belong to the class of genetic variations, which
depend upon the structure or constitution of the protoplasm; but