o avoid confusion, I
will follow established custom, and subsequently speak of this
hypothesis as the Lamarckian hypothesis--understanding, however, that in
employing this designation I am not referring to any part or factor of
Lamarck's general theory of evolution other than the one which has just
been described--namely, the hypothesis of the cumulative transmission of
functionally-produced, or otherwise "acquired," modifications.

This, then, was the earliest hypothesis touching the causes of organic
evolution. But we may at once perceive that it is insufficient to
explain all that stands to be explained. In the first place, it refers
in chief part only to the higher animals, which are actuated to effort
by intelligence. Its explanatory power in the case of most
invertebrata--as well as in that of all plants--is extremely limited,
inasmuch as these organisms can never be moved to a greater or less use
of their several parts by any discriminating volition, such as that
which leads to the continued straining of a giraffe's neck for the
purpose of reaching foliage. In the second place, even among the higher
animals there are numberless tissues and organs which unquestionably
present a high degree of adaptive evolution, but which nevertheless
cannot be supposed to have fallen within the influence of Lamarckian
principles. Of such are the shells of crustacea, tortoises, &c., which
although undoubtedly of great use to the animals presenting them, cannot
ever have been _used_ in the sense required by Lamarck's hypothesis, i.
e. actively exercised, so as to increase a flow of nutrition to the
part. Lastly, in the third place, the validity of Lamarck's hypothesis
in any case whatsoever has of late years become a matter of serious
question, as will be fully shown and discussed in the next volume.
Meanwhile it is enough to observe that, on account of all these reasons,
the theory of Lamarck, even if it be supposed to present any truth at
all, is clearly insufficient as a full or complete theory of organic
evolution.

* * * * *

In historical order the next theory that was arrived at was the theory
of natural selection, simultaneously published by Darwin and Wallace on
July 1st, 1858.

If we may estimate the importance of an idea by the change of thought
which it effects, this idea of natural selection is unquestionably the
most important idea that has ever been conceived by the mind of man.
Ye