178 
ANIMAL PEDIGREES. 
Aug., 1891 . 
tion, then it may be pointed out in reply that sexual repro¬ 
duction is the characteristic and essential mode of multiplica¬ 
tion among Metazoa : that it occurs in all Metazoa, and that 
when asexual reproduction, as by budding, etc., occurs, this 
merely alternates with the sexual process which, sooner or 
later, becomes essential. 
If the fundamental importance of sexual reproduction to 
the welfare of the species be granted, and if it be further 
admitted that Metazoa are descended from Protozoa, then 
we see that there is really a constraining force of a most 
powerful nature compelling every animal to commence its 
life history in the unicellular condition, the only condition 
in which the advantage of cross-fertilisation can be obtained; 
i.e ., constraining every animal to begin its development at its 
earliest ancestral stage, at the very bottom of its genealogical 
tree. 
On this view the actual development of any animal is 
strictly limited at both ends : it must commence as an egg, 
and it must end in the likeness of the parent. The problem 
of recapitulation becomes thereby greatly narrowed ; all that 
remains being to explain why the intermediate stages in the 
actual development should repeat the intermediate stages 
of the ancestral history. 
Although narrowed in this way, the problem still remains 
one of extreme difficulty. 
It is a consequence of the Theory of Natural Selection 
that identity of structure involves community of descent: a 
given result can only be arrived at through a given sequence 
of events : the same morphological goal cannot be reached by 
two independent paths. A negro and a white man have had 
common ancestors in the past; and it is through the long- 
continued action of selection and environment that the two 
types have been gradually evolved. You cannot turn a white 
man into a negro merely by sending him to live in Africa : to 
create a negro the whole ancestral history would have to be 
repeated ; and it may be that it is for the same reason that 
the embryo must repeat or recapitulate its ancestral history 
in order to reach the adult goal. 
I am not sure that we can get much further than this at 
present. 
However, be the explanation what it may, there can, I 
think, be no doubt as to the general truth of the Recapitulation 
Theory, and the wonderful assistance which it gives us in 
reconstructing the pedigrees of animals. Yet it must not be 
supposed that all we have to do in order to determine the 
past history of a species is to study the actual development 
of the existing members of that species. 
