Since his day
they have been re-discovered--or rather re-named--by a host of students,
including Haeckel, Weismann, and many of scarcely less distinction. The
Mendelian "factors," as I maintain must be clear to any student of the
idea, are Spencer's physiological units. Of course neither Spencer nor
any one else, until the re-discovery of Mendel's work, had any notion at
all of the remarkable fashion in which these units are treated in the
process whereby germ-cells are prepared for their great destiny. The
rule, as we now know, is that one germ-cell contains any given unit,
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.
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