different animals; attention was called even then to their close
resemblance to certain fully-developed animal forms belonging to some
of the lower groups. The older scientists (Oken, Treviranus, and
others) knew perfectly well that these lower forms in a sense
illustrated and fixed, in the hierarchy of the animal world, a
temporary stage in the evolution of higher forms. The famous anatomist
Meckel spoke in 1821 of a "similarity between the development of the
embryo and the series of animals." Baer raised the question in 1828
how far, within the vertebrate type, the embryonic forms of the higher
animals assume the permanent shapes of members of lower groups. But it
was impossible fully to understand and appreciate this remarkable
resemblance at that time. We owe our capacity to do this to the theory
of descent; it is this that puts in their true light the action of
heredity on the one hand and adaptation on the other. It explains to
us the vital importance of their constant reciprocal action in the
production of organic forms. Darwin was the first to teach us the
great part that was played in this by the ceaseless struggle for
existence between living things, and to show how, under the influence
of this (by natural selection), new species were produced and
maintained solely by the interaction of heredity and adaptation. It
was thus Darwinism that first opened our eyes to a true comprehension
of the supremely important relations between the two parts of the
science of organic evolution--Ontogeny and Phylogeny.

Heredity and adaptation are, in fact, the two constructive
physiological functions of living things; unless we understand these
properly we can make no headway in the study of evolution. Hence,
until the time of Darwin no one had a clear idea of the real nature
and causes of embryonic development. It was impossible to explain the
curious series of forms through which the human embryo passed; it was