is "the most important
means of modification."

Mr. Darwin gave us no help in this respect; and worse than this, he
contradicted himself so flatly as to show that he had very little
definite idea upon the subject at all. Thus in respect to the
winglessness of the Madeira beetles he wrote:--

"In some cases we might easily put down to disuse modifications of
structure, which are wholly or mainly due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out
of the 550 species (but more are now known) inhabiting Madeira, are
so far deficient in wings that they cannot fly; and that of the 29
endemic genera no less than 23 have all their species in this
condition! Several facts--namely, that beetles in many parts of the
world are frequently blown out to sea and perish; that the beetles
in Madeira, as observed by Mr. Wollaston, lie much concealed until
the wind lulls and the sun shines; that the proportion of wingless
beetles is larger on the exposed Desertas than in Madeira itself;
and especially the extraordinary fact, so strongly insisted on by
Mr. Wollaston, that certain large groups of beetles, elsewhere
excessively numerous, which absolutely require the use of their
wings are here almost entirely absent;--these several considerations
make me believe that the wingless condition of so many Madeira
beetles is mainly due to the action of natural selection, _combined
probably with disuse_ [italics mine]. For during many successive
generations each individual beetle which flew least, either from its
wings having been ever so little less perfectly developed or from
indolent habit, will have had the best chance of surviving, from not
being blown out to sea; and, on the other hand, those beetles which
most readily took to flight would oftenest have been blown to sea,
and thus destroyed." {265}

We should like to know, first, somewhere about how much disuse was
able to do after all, and moreover why, if it can do anything at
all, it should not be able to do all. Mr. Darwin says: "Any change
in structure and function which can be effected by small stages is
within the power of natural selection." "And why not," we ask,
"within the power of use and disuse?" Moreover, on a later page we
find Mr. Darwin saying:--

"_It appears probable that disuse has been the main agent in
rendering organs rudimentary_ [italics mine]. It would at first
lead by slow steps to the more and more complet