mingled
with one another indiscriminately. This is undoubtedly true, as
Romanes pointed out ten years before I did, and this mingling of the
bad with the good probably does bring about a deterioration of the
part concerned. But it cannot account for the steady diminution, which
always occurs when a part is in process of becoming rudimentary, and
which goes on until it ultimately disappears altogether. The process
of dwindling cannot therefore be explained as due to panmixia alone:
we can only find a sufficient explanation in germinal selection.

IV. DERIVATIVES OF THE THEORY OF SELECTION

The impetus in all directions given by Darwin through his theory of
selection has been an immeasurable one, and its influence is still
felt. It falls within the province of the historian of science to
enumerate all the ideas which, in the last quarter of the nineteenth
century, grew out of Darwin's theories, in the endeavour to penetrate
more deeply into the problem of the evolution of the organic world.
Within the narrow limits to which this paper is restricted, I cannot
attempt to discuss any of these.

V. ARGUMENTS FOR THE REALITY OF THE PROCESSES OF SELECTION

(_a_) _Sexual Selection_

Sexual selection goes hand in hand with natural selection. From the
very first I have regarded sexual selection as affording an extremely
important and interesting corroboration of natural selection, but,
singularly enough, it is precisely against this theory that an adverse
judgment has been pronounced in so many quarters, and it is only quite
recently, and probably in proportion as the wealth of facts in proof
of it penetrates into a wider circle, that we seem to be approaching a
more general recognition of this side of the problem of adaptations.
Thus Darwin's words in his preface to the second edition (1874) of his
book, _The Descent of Man and Sexual Selection_, are being justified:
"My conviction as to the operation of natural selection remains
unshaken," and further, "If naturalists were to become more familiar
with the idea of sexual selection, it would, I think, be accepted to a
much greater extent, and already it is fully and favourably accepted
by many competent judges." Darwin was able to speak thus because he
was already acquainted with an immense mass of facts, which, taken
together, yield overwhelming evidence of the validity of the principle
of sexual selection.

_Natural selection_ chooses out for reproduction the indiv