this fundamental law of organic evolution holds good generally, and
that there is everywhere a direct causal connection between ontogeny
and phylogeny. "Phylogenesis is the mechanical cause of ontogenesis"; in
other words, "The evolution of the stem or race is--in accordance with
the laws of heredity and adaptation--the real cause of all the changes
that appear, in a condensed form, in the development of the individual
organism from the ovum, in either the embryo or the larva."

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." ("Origin of Species" (6th edition), page 396.)

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 Muller made a closer study of these
important phenomena in the instructive instance of the Crustacean larva,
as given in his able work "Fur Darwin" (1864). (English translation;
"Facts and Arguments for Darwin", London, 1869.) 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". (3 volumes,
Berlin, 1894-96.) It has, however, been frequently chall