es in the fact that the former can
modify only a few characters, usually only one at a time, while Nature
preserves in the struggle for existence all the variations of a species,
at the same time and in a purely mechanical way, if they possess
selection-value.

Herbert Spencer, though himself an adherent of the theory of selection,
declared in the beginning of the nineties that in his opinion the range
of this principle was greatly over-estimated, if the great changes which
have taken place in so many organisms in the course of ages are to
be interpreted as due to this process of selection alone, since no
transformation of any importance can be evolved by itself; it is always
accompanied by a host of secondary changes. He gives the familiar
example of the Giant Stag of the Irish peat, the enormous antlers of
which required not only a much stronger skull cap, but also greater
strength of the sinews, muscles, nerves and bones of the whole anterior
half of the animal, if their mass was not to weigh down the animal
altogether. It is inconceivable, he says, that so many processes of
selection should take place SIMULTANEOUSLY, and we are therefore
forced to fall back on the Lamarckian factor of the use and disuse of
functional parts. And how, he asks, could natural selection follow two
opposite directions of evolution in different parts of the body at the
same time, as for instance in the case of the kangaroo, in which the
forelegs must have become shorter, while the hind legs and the tail were
becoming longer and stronger?

Spencer's main object was to substantiate the validity of the Lamarckian
principle, the cooperation of which with selection had been doubted
by many. And it does seem as though this principle, if it operates
in nature at all, offers a ready and simple explanation of all such
secondary variations. Not only muscles, but nerves, bones, sinews,
in short all tissues which function actively, increase in strength
in proportion as they are used, and conversely they decrease when the
claims on them diminish. All the parts, therefore, which depend on the
part that varied first, as for instance the enlarged antlers of the
Irish Elk, must have been increased or decreased in strength, in exact
proportion to the claims made upon them,--just as is actually the case.

But beautiful as this explanation would be, I regard it as untenable,
because it assumes the TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS
(so-called "ac