ier sketch of his theory of evolution
(1844) he attached more weight to the inheritance of acquired habits
than he does in his _Origin of Species_ published fifteen years
later.[18] He appears to have acquired the belief in early life without
first questioning and rigorously testing it as he would have done had it
originated with himself. In later life it appeared to assist his theory
of evolution in minor points, and in particular it appeared absolutely
indispensable to him as the _only_ explanation of the diminution of
disused parts in cases where, as in domestic animals, economy of growth
seemed to be practically powerless. He failed to adequately notice the
effect of panmixia, or the withdrawal of selection, in causing or
allowing degeneracy and dwindling under disuse; and he hardly attached
sufficient importance to the fact that rudimentary organs and other
supposed effects of use or disuse are quite as marked features in
neuter insects which cannot transmit the effects of use and disuse as
they are in the higher animals.

REDUCED WINGS OF BIRDS OF OCEANIC ISLANDS.

Darwin himself has pointed out that the rudimentary wings of island
beetles, at first thought to be due to disuse, are mainly brought about
by natural selection--the best-winged beetles being most liable to be
blown out to sea. But he says that in birds of the oceanic islands "not
persecuted by any enemies, the reduction of their wings has probably
been caused by disuse." This explanation may be as fallacious as it is
acknowledged to have been in the case of the island beetles. According
to Darwin's own views, natural selection _must_ at least have played an
important part in reducing the wings; for he holds that "natural
selection is continually trying to economize every part of the
organization." He says: "If under changed conditions of life a
structure, before useful, becomes less useful, its diminution will be
favoured, for it will profit the individual not to have its nutriment
wasted in building up an useless structure.... Thus, as I believe,
natural selection will tend in the long run to reduce any part of the
organization, as soon as it becomes, through changed habits,
superfluous."[19] If, as Darwin powerfully urges (and he here ignores
his usual explanation), ostriches' wings are insufficient for flight in
consequence of the economy enforced by natural selection,[20] why may
not the reduced wings of the dodo, or the penguin, or the apteryx,