VON BAER'S LAW 141

contributions, as a multiplex of potentialities; it is
even visibly very complex and anything but homo-
geneous or *< simple**; and the individual recapitnla-
tion of racial history is verifiable rather in the stages
of organogenesis than in the history of the embryo
as a whole. Thus while all are agreed that there
is a gradual emergence of the obviously complex
from the apparently simple, that development means
progressive differentiation and integration, and that
past history is in some measure resumed in present
development, it must also be allowed that germ-cells
are microcosms of complexity, that development is
the realisation of a composite inheritance, the cashing
of ancestral cheques, and that the "minting and
coining of the chick out of the egg " is not adequately
summed up as "a progress from homogeneity to
heterogeneity/*

But although embryology does not appear to us
to give unequivocal support to Spencer's formula of
progress from the homogeneous to the heterogeneous,
it seemed all plain sailing to him, and he proceeded
to illustrate the utility of his formula by applying it
to all orders of facts. In a famous passage in the
essay on "Progress: its Law and Cause" (Essays,
Vol. L, 1883, p. 30) he wrote as follows:—

" We believe we have shown beyond question that that
which the German physiologists (von Baer, Wolff, and
others) have found to be the law of organic development
(as of a seed into a tree and of an egg into an animal) is
the law of all development. The advance from the simple to
the complex, through a process of successive differentiations
(i.e. the appearance of differences in the parts of a seemingly
like substance), is seen alike in the earliest changes of the
Universe to which we can reason our way back; and inght be looked for, did sperm-cells and germ-cellsestion.