fertilization would cause the number of chromosomes in the fertilized
ovum to be double the number characteristic of the species. In man,
for example, the addition of twenty-four chromosomes from the
spermatozoon to an ovum that already contained twenty-four
chromosomes of its own would mean that after fertilization the ovum
contained forty-eight. Such a result is prevented through the process
to which we have referred in the preceding chapter as the ripening of
the ovum, and also through a similar process in the case of the
spermatozoon. These two processes lead to a reduction in the number
of chromosomes, so that finally every human germinal cell contains
twelve, and therefore when the ovum is fertilized the characteristic
number twenty-four is restored. While we know nothing of the forces
which determine, on the one hand, what elements shall be discarded by
the germinal cells and, on the other hand, what elements shall
remain, it is definitely proved that a selective process always takes
place. This fact admirably explains the variation in the
characteristics inherited by children of the same family. So far as
is known, the traits which will be passed on from either parent are a
matter of chance. Whatever these hereditary traits happen to be, the
best evidence we have indicates that the problem of a child's
inheritance is settled once for all the moment conception takes
place.

MATERNAL IMPRESSIONS.--Contrary to all that we know of heredity, the
conviction prevails among the laity that the character of a child
depends greatly upon the mother's surroundings during pregnancy: this
is the doctrine of maternal impressions. As is usual with
superstitions, this one emphasizes the unfavorable possibilities and
holds that the unborn child may be affected by the mother's unhappy
thoughts or maimed by her mental distress if she is exposed to