256 Darwin, and after Darwin. 
effort by intelligence. Its explanatory power in the 
case of most invertebrata—as well as in that ofall 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 unques- 
tionably 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 wsed 
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 in- 
sufficient 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 ist, 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 
