advance of embryology as this; its formulation is one of the most
signal services to general biology. It was not until this law passed
into the flesh and blood of investigators, and they had accustomed
themselves to see a reminiscence of ancestral history in embryonic
structures, that we witnessed the great progress which embryological
research has made in the last two decades." The best proof of the
correctness of this opinion is that now the most fruitful work is done
in all branches of embryology with the aid of this biogenetic law, and
that it enables students to attain every year thousands of brilliant
results that they would never have reached without it.

It is only when one appreciates the cenogenetic processes in relation
to the palingenetic, and when one takes careful account of the changes
which the latter may suffer from the former, that the radical
importance of the biogenetic law is recognised, and it is felt to be
the most illuminating principle in the science of evolution. In this
task of discrimination it is the silver thread in relation to which we
can arrange all the phenomena of this realm of marvels--the "Ariadne
thread," which alone enables us to find our way through this labyrinth
of forms. Hence the brothers Sarasin, the zoologists, could say with
perfect justice, in their study of the evolution of the Ichthyophis,
that "the great biogenetic law is just as important for the zoologist
in tracing long-extinct processes as spectrum analyses is for the
astronomer."

Even at an earlier period, when a correct acquaintance with the
evolution of the human and animal frame was only just being
obtained--and that is scarcely eighty years ago!--the greatest
astonishment was felt at the remarkable similarity observed between
the embryonic forms, or stages of foetal development, in very