42 The Selection Theory 
convinced, in the course of years, that the Lamarckian principle 
ought not to be called in to explain the dwindling of disused parts, 
I believed that this process might be simply explained as due to 
the cessation of the conservative effect of natural selection. I said to 
myself that, from the moment in which a part ceases to be of use, 
natural selection withdraws its hand from it, and then it must 
inevitably fall from the height of its adaptiveness, because inferior 
variants would have as good a chance of persisting as better ones, 
since all grades of fitness of the part in question would be 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 rudi- 
mentary, 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 ex- 
tremely 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 adaptation. Thus Darwin’s words in his preface to the second 
edition (1874) of his book, The Descent of Man and Sexual Selection, 
