of man and the mammals by a proper comparison of the embryology of
very different animals--a picture that we could never have framed from
the ontogeny of the mammals alone. As a result of the above-mentioned
cenogenetic processes--those of disturbed and curtailed
heredity--whole series of lower stages have dropped out in the
embryonic development of man and the other mammals especially from the
earliest periods, or been falsified by modification. But we find these
lower stages in their original purity in the lower vertebrates and
their invertebrate ancestors. Especially in the lowest of all the
vertebrates, the lancelet or Amphioxus, we have the oldest stem-forms
completely preserved in the embryonic development. We also find
important evidence in the fishes, which stand between the lower and
higher vertebrates, and throw further light on the course of evolution
in certain periods. Next to the fishes come the amphibia, from the
embryology of which we can also draw instructive conclusions. They
represent the transition to the higher vertebrates, in which the
middle and older stages of ancestral development have been either
distorted or curtailed, but in which we find the more recent stages of
the phylogenetic process well preserved in ontogeny. We are thus in a
position to form a fairly complete idea of the past development of
man's ancestors within the vertebrate stem by putting together and
comparing the embryological developments of the various groups of
vertebrates. And when we go below the lowest vertebrates and compare
their embryology with that of their invertebrate relatives, we can
follow the genealogical tree of our animal ancestors much farther,
down to the very lowest groups of animals.

In entering the obscure paths of this phylogenetic labyrinth, clinging
to the Ariadne-thread of the biogenetic law and guided by the light of
comparative anatomy, we will first, in accordance with the methods we