strong presumption. Again, it is easy to over-value such complex
instruments as we possess. The possessor of an up-to-date camera,
well instructed in the function and manipulation of every part, but
ignorant of all optics save a hand-to-mouth knowledge of the
properties of his own lens, might say that a priori no picture could
be taken with a cigar-box perforated by a pin-hole; and our ignorance
of the mechanism of the Psychology of any organism is greater by many
times than that of my supposed photographer. We know that Plants are
able to do many things that can only be accounted for by ascribing to
them a "psyche," and these co-ordinated enough to satisfy their
needs; and yet they possess no central organ comparable to the brain,
no highly specialised system for intercommunication like our nerve
trunks and fibres. As Oscar Hertwig says, we are as ignorant of the
mechanism of the development of the individual as we are of that of
hereditary transmission of acquired characters, and the absence of
such mechanism in either case is no reason for rejecting the proven
fact.

However, the relations of germ and body just described led Jager,
Nussbaum, Galton, Lankester, and, above all, Weismann, to the view
that the germ-cells or "stirp" (Galton) were IN the body, but not OF
it. Indeed, in the body and out of it, whether as reproductive cells
set free, or in the developing embryo, they are regarded as forming
one continuous homogeneity, in contrast to the differentiation of the
body; and it is to these cells, regarded as a continuum, that the
terms stirp, germ-plasm, are especially applied. Yet on this view,
so eagerly advocated by its supporters, we have to substitute for the
hypothesis of memory, which they declare to have no real meaning
here, the far more fantastic hypotheses of Weismann: by these they
explain the process of differentiation in the young embryo into new