enged, 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, are due to heredity, to the transmission of characters
from one generation to another. They enable us to draw direct inferences
in regard to corresponding structures in the development of the species
(e.g. the chorda or the branchial arches in all vertebrate embryos). The
cenogenetic phenomena, on the other hand, or the embryonic VARIATIONS,
cannot be traced to inheritance from a mature ancestor, but are due to
the adaptation of the embryo or the larva to certain conditions of
its individual development (e.g. the amnion, the allantois, and the
vitelline arteries in the embryos of the higher vertebrates). These
cenogenetic phenomena are later additions; we must not infer from them
that there were corresponding processes in the ancestral history, and
hence they are apt to mislead."

The fundamental importance of these facts of comparative anatomy,
atavism, and the rudimentary organs, was pointed out by Darwin in the
first part of his classic work, "The Descent of Man and Selection in
Relation to Sex" (1871). ("Descent of Man" (Popular Edition), page 927.)
In the "General summary and conclusion" (chapter XXI.) he was able
to say, with perfect justice: "He who is not content to look, like a
savage, at the phenomena of nature as disconnected, cannot any longer
believe that man is the work of a separate act of creation. He will be
forced to admit that the close resemblance of the embryo of man to that,
for instance, of a dog--the construction of his skull, limbs, and whole
frame on the same plan with that of other mammals, independently of
the uses to which the parts may be put--the occasional reappearance of
various structures, for instance of several muscles, which man does not
normally possess, but which are common to the Quadrumana--and a crowd of
analogous facts--all point in the plainest manner to the conclusion that
man i