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.
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