for reasons that are as yet only partly known to us. The fact is
easily proved by comparing the different developments of higher and
lower animals in any single stem.

In order to appreciate this important feature, we have distributed the
embryological phenomena in two groups, palingenetic and cenogenetic.
Under palingenesis we count those facts of embryology that we can
directly regard as a faithful synopsis of the corresponding
stem-history. By cenogenesis we understand those embryonic processes
which we cannot directly correlate with corresponding evolutionary
processes, but must regard as modifications or falsifications of them.
With this careful discrimination between palingenetic and cenogenetic
phenomena, our biogenetic law assumes the following more precise
shape:--The rapid and brief development of the individual (ontogeny)
is a condensed synopsis of the long and slow history of the stem
(phylogeny): this synopsis is the more faithful and complete in
proportion as the original features have been preserved by heredity,
and modifications have not been introduced by adaptation.

In order to distinguish correctly between palingenetic and cenogenetic
phenomena in embryology, and deduce sound conclusions in connection
with stem-history, we must especially make a comparative study of the
former. In doing this it is best to employ the methods that have long
been used by geologists for the purpose of establishing the succession
of the sedimentary rocks in the crust of the earth. This solid crust,
which encloses the glowing central mass like a thin shell, is composed
of different kinds of rocks: there are, firstly, the volcanic rocks
which were formed directly by the cooling at the surface of the molten
mass of the earth; secondly, there are the sedimentary rocks, that
have been made out of the former by the action of water, and have been