In the first chapter I attempted to show that
changed conditions act in two ways, directly on the whole organisation or
on certain parts alone, and indirectly through the reproductive system. In
all cases there are two factors, the nature of the organism, which is much
the most important of the two, and the nature of the conditions. The
direct action of changed conditions leads to definite or indefinite
results. In the latter case the organisation seems to become plastic, and
we have much fluctuating variability. In the former case the nature of the
organism is such that it yields readily, when subjected to certain
conditions, and all, or nearly all, the individuals become modified in the
same way.
It is very difficult to decide how far changed conditions, such as of
climate, food, etc., have acted in a definite manner. There is reason to
believe that in the course of time the effects have been greater than can
be proved by clear evidence. But we may safely conclude that the
innumerable complex co-adaptations of structure, which we see throughout
nature between various organic beings, cannot be attributed simply to such
action. In the following cases the conditions seem to have produced some
slight definite effect: E. Forbes asserts that shells at their southern
limit, and when living in shallow water, are more brightly coloured than
those of the same species from further north or from a greater depth; but
this certainly does not always hold good.
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