The Evolution of Man, vol 2 | Page 3

the conditions of existence, and
have lost their original character to some extent. During the
immeasurable course of organic history, the many millions of years
during which life was developing on our planet, secondary changes of
the embryonic forms have taken place in most animals. The young of
animals (not only detached larvae, but also the embryos enclosed in the
womb) may be modified by the influence of the environment, just as
well as the mature organisms are by adaptation to the conditions of life;
even species are altered during the embryonic development. Moreover,
it is an advantage for all higher organisms (and the advantage is greater
the more advanced they are) to curtail and simplify the original course
of development, and thus to obliterate the traces of their ancestors. The
higher the individual organism is in the animal kingdom, the less
completely does it reproduce in its embryonic development the series
of its ancestors, 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 laid in successive strata at the bottom of the sea. Each of these
sedimentary strata was at first a soft layer of mud; but in the course of
thousands of years it condensed into a solid, hard mass of stone
(sandstone, limestone, marl, etc.), and at the same time permanently
preserved the solid and imperishable bodies that had chanced to fall
into the soft mud. Among these bodies, which were either fossilised or
left characteristic impressions of their forms in the soft slime, we have
especially the more solid parts of the animals and plants that lived and
died during the deposit of the slimy strata.
Hence each of the sedimentary strata has its characteristic fossils, the
remains of the animals and plants that lived during that particular
period of the earth's history. When we make a comparative study of
these strata, we can survey the whole series of such periods. All
geologists are now agreed that we can demonstrate a definite historical

succession in the strata, and that the lowest of them were deposited in
very remote, and the uppermost in comparatively recent, times.
However, there is no part of the earth where we find the series of strata
in its entirety, or even approximately complete. The succession of strata
and of corresponding historical periods generally given in geology is an
ideal construction, formed by piecing together the various partial
discoveries of the succession of strata that have been made at different
points of the earth's surface (cf.
Chapter 2.
18).
We must act in this way in constructing the phylogeny of man. We
must try to piece together a fairly complete picture of the series of our
ancestors from the various phylogenetic fragments that we find in the
different groups of the animal kingdom. We shall see that we are really
in a position to form an approximate picture of the evolution of man
and the mammals by a proper comparison of the embryology of very
different animals--a picture that we could never have framed from the
ontogeny of the mammals alone. As a result of the above-mentioned
cenogenetic processes--those of disturbed and curtailed