[Gutenberg 6710] • The Evolution of Man — Volume 2

Excerpt from the book...

In turning from the embryology to the phylogeny of man--from the

development of the individual to that of the species--we must bear in

mind the direct causal connection that exists between these two main

branches of the science of human evolution. This important causal

nexus finds its simplest expression in "the fundamental law of organic

development," the content and purport of which we have fully

considered in the first chapter. According to this biogenetic law,

ontogeny is a brief and condensed recapitulation of phylogeny. If this

compendious reproduction were complete in all cases, it would be very

easy to construct the whole story of evolution on an embryonic basis.

When we wanted to know the ancestors of any higher organism, and,

therefore, of man--to know from what forms the race as a whole has

been evolved we should merely have to follow the series of forms in

the development of the individual from the ovum; we could then regard

each of the successive forms as the representative of an extinct

ancestral form. However, this direct application of ontogenetic facts

to phylogenetic ideas is possible, without limitations, only in a very

small section of the animal kingdom. There are, it is true, still a

number of lower invertebrates (for instance, some of the Zoophyta and

Vermalia) in which we are justified in recognising at once each

embryonic form as the historical reproduction, or silhouette, as it

were, of an extinct ancestor. But in the great majority of the

animals, and in the case of man, this is impossible, because the

embryonic forms themselves have been modified through the change of

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.