uality of its possessor, and the same individuality
moulds similarly and simultaneously the germ-cells. Both are
different expressions or manifestations of the same individuality.
Only slowly does the individuality mould the muscles and nerves of
the adult body to its use. Still more slow may be the moulding of
the still more refractory germ-plasm, if such there be. But the
moulding process goes on parallel in the two cases.

But Weismann's argument rests not merely upon any difficulty or
impossibility of the transmissibility of acquired characteristics.
His argument is rather that all facts can be better explained by his
theory without postulating or accepting such transmission, cases of
which have never been absolutely proven. But the question is not
whether his theory offers a possible explanation of the facts, but
whether it is the most probable explanation of all the facts. No one
would deny, I think, that the continuity of the germ-plasm offers
the best and most natural explanation of heredity; and that
variations could be produced by the influence on the germ-plasm of
external conditions seems entirely probable.

But when we consider the aggregation of these variations in a
process of evolution, his theory seems unsatisfactory. We have
already seen that what we commonly call a variation involves not one
change, but a series of changes, each term of which is necessary.
Muscle, nerve, and ganglion must all vary simultaneously and
correspondingly. Correlation and combination are just as essential
as variation. And evolution often demands the disappearance of less
fit structures just as much as the advance of the fittest. Says
Osborne, "It is misleading to base our theory of evolution and
heredity solely upon entire organs; in the hand and foot we have
numerous cases of muscles in close contiguity, one steadily
developing, the other degenerating." Weismann offers the explanation
that "if the average amount of food which an animal can assimilate
every day remains constant for a considerable time, it follows that
a strong influx toward one organ must be accompanied by a drain upon
others, and this tendency will increase, from generation to
generation, in proportion to the development of the growing organ,
which is favored by natural selection in its increased blood-supply,
etc.; while the operation of natural selection has also determined
the organ which can bear a corresponding loss without detriment to
the organis