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