fundamental law of biogeny? If we are impartial, we must reply that it
has proved its fertility in hundreds of sound results, and that by its
aid we have acquired a vast fund of knowledge which we should never
have obtained without it.

There has been no dearth of attacks--often violent attacks--on my
conception of an intimate causal connection between ontogenesis and
phylogenesis; but no other satisfactory explanation of these important
phenomena has yet been offered to us. I say this especially with
regard to Wilhelm His's theory of a "mechanical evolution," which
questions the truth of phylogeny generally, and would explain the
complicated embryonic processes without going beyond by simple
physical changes--such as the bending and folding of leaves by
electricity, the origin of cavities through unequal strain of the
tissues, the formation of processes by uneven growth, and so on. But
the fact is that these embryological phenomena themselves demand
explanation in turn, and this can only be found, as a rule, in the
corresponding changes in the long ancestral series, or in the
physiological functions of heredity and adaptation.


CHAPTER 1.2. THE OLDER EMBRYOLOGY.

It is in many ways useful, on entering upon the study of any science,
to cast a glance at its historical development. The saying that
"everything is best understood in its growth" has a distinct
application to science. While we follow its gradual development we get
a clearer insight into its aims and objects. Moreover, we shall see
that the present condition of the science of human evolution, with all
its characteristics, can only be rightly understood when we examine