."

It is now fifty years since Charles Darwin pointed out, in the
thirteenth chapter of his epoch-making _Origin of Species_, the
fundamental importance of embryology in connection with his theory of
descent:

"The leading facts in embryology, which are second to none in
importance, are explained on the principle of variations in the many
descendants from some one ancient progenitor, having appeared at a not
very early period of life, and having been inherited at a
corresponding period."[137]

He then shows that the striking resemblance of the embryos and larvae
of closely related animals, which in the mature stage belong to widely
different species and genera, can only be explained by their descent
from a common progenitor. Fritz Mueller made a closer study of these
important phenomena in the instructive instance of the Crustacean
larva, as given in his able work _Fuer Darwin_[138] (1864). I then, in
1872, extended the range so as to include all animals (with the
exception of the unicellular Protozoa) and showed, by means of the
theory of the Gastraea, that all multicellular, tissue-forming
animals--all the Metazoa--develop in essentially the same way from the
primary germ-layers.

I conceived the embryonic form, in which the whole structure consists of
only two layers of cells, and is known as the gastrula, to be the
ontogenetic recapitulation, maintained by tenacious heredity, of a
primitive common progenitor of all the Metazoa, the Gastraea. At a later
date (1895) Monticelli discovered that this conjectural ancestral form is
still preserved in certain primitive Coelenterata--Pemmatodiscus,
Kunstleria, and the nearly-related Orthonectida.

The general application of the biogenetic law to all classes of
animals and plants has been proved in my _Systematische
Phylogenie_.[139] It has, however, been frequently challenged, both by
botanists and zoologists, chiefly owing to the fact that many have
failed to distinguish its two essential elements, palingenesis and
cenogenesis. As early as 1874 I had emphasised, in the first chapter
of my _Evolution of Man_, the importance of discriminating carefully
between these two sets of phenomena:

"In the evolutionary appreciation of the facts of embryology we must
take particular care to distinguish sharply and clearly between the
primary, palingenetic evolutionary processes and the secondary,
cenogenetic processes. The palingenetic phenomena, or embryonic
_recapitulations