compendious reproduction were complete in all cases, it would be very
easy to construct the whole story of evolution on an embryonic basis.
When we wanted to know the ancestors of any higher organism, and,
therefore, of man--to know from what forms the race as a whole has
been evolved we should merely have to follow the series of forms in
the development of the individual from the ovum; we could then regard
each of the successive forms as the representative of an extinct
ancestral form. However, this direct application of ontogenetic facts
to phylogenetic ideas is possible, without limitations, only in a very
small section of the animal kingdom. There are, it is true, still a
number of lower invertebrates (for instance, some of the Zoophyta and
Vermalia) in which we are justified in recognising at once each
embryonic form as the historical reproduction, or silhouette, as it
were, of an extinct ancestor. But in the great majority of the
animals, and in the case of man, this is impossible, because the
embryonic forms themselves have been modified through the change of
the conditions of existence, and have lost their original character to
some extent. During the immeasurable course of organic history, the
many millions of years during which life was developing on our planet,
secondary changes of the embryonic forms have taken place in most
animals. The young of animals (not only detached larvae, but also the
embryos enclosed in the womb) may be modified by the influence of the
environment, just as well as the mature organisms are by adaptation to
the conditions of life; even species are altered during the embryonic
development. Moreover, it is an advantage for all higher organisms
(and the advantage is greater the more advanced they are) to curtail
and simplify the original course of development, and thus to
obliterate the traces of their ancestors. The higher the individual
organism is in the animal kingdom, the less completely does it
reproduce in its embryonic development the series of its ancestors,