1.26 and 1.27) indicates the precise moment at which the individual
begins to exist. All the bodily and mental features of the new-born
child are the sum-total of the hereditary qualities which it has
received in reproduction from parents and ancestors. All that man
acquires afterwards in life by the exercise of his organs, the
influence of his environment, and education--in a word, by
adaptation--cannot obliterate that general outline of his being which
he inherited from his parents. But this hereditary disposition, the
essence of every human soul, is not "eternal," but "temporal"; it
comes into being only at the moment when the sperm-nucleus of the
father and the nucleus of the maternal ovum meet and fuse together. It
is clearly irrational to assume an "eternal life without end" for an
individual phenomenon, the commencement of which we can indicate to a
moment by direct visual observation.

The great importance of the process of impregnation in answering such
questions is quite clear. It is true that conception has never been
studied microscopically in all its details in the human
case--notwithstanding its occurrence at every moment--for reasons that
are obvious enough. However, the two cells which need consideration,
the female ovum and the male spermatozoon, proceed in the case of man
in just the same way as in all the other mammals; the human foetus or
embryo which results from copulation has the same form as with the
other animals. Hence, no scientist who is acquainted with the facts
doubts that the processes of impregnation are just the same in man as
in the other animals.

The stem-cell which is produced, and with which every man begins his
career, cannot be distinguished in appearance from those of other
mammals, such as the rabbit (Figure 1.28). In the case of man, also,