that when a marked variation, such as an extra digit, or a reduced
limb, or an extra segment, makes its appearance, it is not confined to
the adult but can be seen all through the development? All the clear
evidence we can get tends to show that marked variations, whether of
reduction or increase, of organs are manifest during the whole of the
development of the organ and do not merely affect the adult. And on
reflection we see that it could hardly be otherwise. All such evidence
is distinctly at variance with the theory of recapitulation, at least
as applied to embryos. In the case of larvae of course the case will be
different, for in them the organs are functional, and reduction in the
adult will not be accompanied by reduction in the larva unless a change
in the conditions of life of the larva enables it to occur.

If after 50 years of research and close examination of the facts of
embryology the recapitulation theory is still without satisfactory
proof, it seems desirable to take a wider sweep and to inquire whether
the facts of embryology cannot be included in a larger category.

As has been pointed out by Huxley, development and life are
co-extensive, and it is impossible to point to any period in the life of
an organism when the developmental changes cease. It is true that these
changes take place more rapidly at the commencement of life, but they
are never wholly absent, and those which occur in the later or so-called
adult stages of life do not differ in their essence, however much they
may differ in their degree, from those which occur during the embryonic
and larval periods. This consideration at once brings the changes of
the embryonic period into the same category as those of the adult and
suggests that an explanation which will account for the one will account
for the other. What then is the problem we are dealing with? Surely
it is this: Why does an organism as soon as it is established at the
fertilisation of the ovum enter upon a cycle of transformations which
never cease until death puts an end to them? In other words what is
the meaning of that cycle of changes which all organisms present in a
greater or less degree and which constitute the very essence of life?
It is impossible to give an answer to this question so long as we remain
within the precincts of Biology--and it is not my present purpose to
penetrate beyond those precincts into the realms of philosophy. We have
to do with an ultimate biolo