The Evolution of Man, vol 1 | Page 9

of manifold animal forms which represent the ancestry of
each higher organism, or even of man, according to the theory of
descent, always form a connected whole. We may designate this
uninterrupted series of forms with the letters of the alphabet: A, B, C, D,
E, etc., to Z. In apparent contradiction to what I have said, the story of
the development of the individual, or the ontogeny of most organisms,
only offers to the observer a part of these forms; so that the defective
series of embryonic forms would run: A, B, D, F, H, K, M, etc.; or, in
other cases, B, D, H, L, M, N, etc. Here, then, as a rule, several of the
evolutionary forms of the original series have fallen out. Moreover, we
often find--to continue with our illustration from the alphabet--one or
other of the original letters of the ancestral series represented by
corresponding letters from a different alphabet. Thus, instead of the
Roman B and D, we often have the Greek Beta and Delta. In this case
the text of the biogenetic law has been corrupted, just as it had been
abbreviated in the preceding case. But, in spite of all this, the series of
ancestral forms remains the same, and we are in a position to discover
its original complexion.
In reality, there is always a certain parallel between the two
evolutionary series. But it is obscured from the fact that in the
embryonic succession much is wanting that certainly existed in the
earlier ancestral succession. If the parallel of the two series were
complete, and if this great fundamental law affirming the causal
connection between ontogeny and phylogeny in the proper sense of the
word were directly demonstrable, we should only have to determine, by
means of the microscope and the dissecting knife, the series of forms
through which the fertilised ovum passes in its development; we should
then have before us a complete picture of the remarkable series of

forms which our animal ancestors have successively assumed from the
dawn of organic life down to the appearance of man. But such a
repetition of the ancestral history by the individual in its embryonic life
is very rarely complete. We do not often find our full alphabet. In most
cases the correspondence is very imperfect, being greatly distorted and
falsified by causes which we will consider later. We are thus, for the
most part, unable to determine in detail, from the study of its
embryology, all the different shapes which an organism's ancestors
have assumed; we usually--and especially in the case of the human
foetus--encounter many gaps. It is true that we can fill up most of these
gaps satisfactorily with the help of comparative anatomy, but we
cannot do so from direct embryological observation. Hence it is
important that we find a large number of lower animal forms to be still
represented in the course of man's embryonic development. In these
cases we may draw our conclusions with the utmost security as to the
nature of the ancestral form from the features of the form which the
embryo momentarily assumes.
To give a few examples, we can infer from the fact that the human
ovum is a simple cell that the first ancestor of our species was a tiny
unicellular being, something like the amoeba. In the same way, we
know, from the fact that the human foetus consists, at the first, of two
simple cell-layers (the gastrula), that the gastraea, a form with two such
layers, was certainly in the line of our ancestry. A later human
embryonic form (the chordula) points just as clearly to a worm-like
ancestor (the prochordonia), the nearest living relation of which is
found among the actual ascidiae. To this succeeds a most important
embryonic stage (acrania), in which our headless foetus presents, in the
main, the structure of the lancelet. But we can only indirectly and
approximately, with the aid of comparative anatomy and ontogeny,
conjecture what lower forms enter into the chain of our ancestry
between the gastraea and the chordula, and between this and the
lancelet. In the course of the historical development many intermediate
structures have gradually fallen out, which must certainly have been
represented in our ancestry. But, in spite of these many, and sometimes
very appreciable, gaps, there is no contradiction between the two
successions. In fact, it is the chief purpose of this work to prove the real
harmony and the original parallelism of the two. I hope to show, on a

substantial basis of facts, that we can draw most important conclusions
as to our genealogical tree from the actual and easily-demonstrable
series of embryonic changes. We shall then be in a position to form a
general idea of the wealth of animal forms which have figured