nit,
while another does not. The process of cell-division, whereby the
germ-cells or gametes[5] are made, is called gameto-genesis. Somewhere
in its course there occurs the capital fact discovered by Mendel and
called by him segregation. A cell divides into two--which are the final
gametes. One of these will definitely contain the Mendelian factor, and
the other will be as definitely without it. Definite consequences follow
in the constitution of the offspring; and such is the Mendelian
contribution to heredity. But we must see that these inquiries cannot be
far pursued without telling us vastly more than we ever knew before of
not only the relation between individuals of successive generations, but
the very structure of the individuals themselves. It is by the study of
heredity that we shall learn to understand the individual. For instance,
experimental breeding of the fowl reveals the existence of the brooding
instinct as a definite unit, which enters, or does not enter, into the
composition of the individual, and which is quite distinct from the
capacity to produce eggs. Here is a definite distinction suggested, for
the case of the fowl, between two really distinct things which, for
several years past, I have called respectively physical and psychical
motherhood. The analysis will doubtless go far further, but already the
facts of experiment help us to realize the composition of the individual
mother--for instance, the number of possible variants, and the
non-necessity of a connection between the capacity to produce children
and the parental instinct upon which the care of them depends, and
without which entire and perfect motherhood cannot be.

The Mendelians are teaching us, too, that their "factors," the units of
which we are made, are often intertangled or mutually repellent. If
such-and-such goes into the germ-cell, so must something else; or if the
one, then never the other. There may thus be naturally determined
conditions of entire womanhood; just as one may be externally a woman,
yet lack certain of the fractional constituents which are necessary for
the perfect being. Complete womanhood, like genius--rarer though not
more valuable--depends upon the co-existence of _many_ factors, some of
which may be coupled and segregated together in gameto-genesis, while
others may be quite independent, only chance determining the throw of
them. And the question of incompatibility or mutual repulsion of factors
is of the