PROBLEMS OF HEREDITY 155

through undifferentiating cell-divisions with the
original germ-cell from which the parental body
developed.

Even in ancient times men pondered over the re-
semblances and differences between children and their
parents—for like only tends to beget like — and
wondered as to the nature of the bond which links
generation to generation. But although the problems
are old, the precise study of them is altogether
modern. The first great step towards clearness was
the formulation of the cell-theory by Schwann and
Schleiden (1838-9), by Goodsir and Virchow, which
made it clear that all but the simplest organisms are
built up of cells or modifications of cells, and that the
individual life usually begins as a fertilised egg-cell
which proceeds by division and re-division, by differ-
entiation and integration, to develop a more or less
complex " body." It has become gradually clear that
while the fertilised egg-cell gives rise to body-cells
which become specialised, it also gives rise to un-
specialised descendant-cells, which take no share in
body-making, but become the germ-cells—the potential
starting-points of another generation. A second
great step was the accumulation of facts of inheritance
showing that all sorts of qualities innate or inborn in
the parents, essential and trivial, normal and abnormal,
bodily and mental, may be transmitted to the offspring
as part of the organic heritage. A third great step
was implied in the acceptance which Darwin in par-
ticular won for the general idea of descent, for it is
hardly too much to say that the scientific study of the
problems of heredity began when it was recognised
that heredity is a fundamental condition of evolution* scientific and trans-s intrinsic incoherence; worthless as