176 Darwin and Embryology 
is negative. The recapitulation theory originated as a deduction 
from the evolution theory and as a deduction it still remains. 
Let us before leaving the subject apply another test. If the 
evolution theory and the recapitulation theory are both true, how 
is it that living birds are not only without teeth but have no rudiments 
of teeth at any stage of their existence? How is it that the missing 
digits in birds and mammals, the missing or reduced limb of snakes 
and whales, the reduced mandibulo-hyoid cleft of elasmobranch fishes 
are not present or relatively more highly developed in the embryo 
than in the adult? How is it 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 accom- 
panied 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 
eo-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 
