1898] SPECIES, SEX, AND THE INDIVIDUAL 187 
the scales in the north than in the south. The difficulty is that this 
effect of northern latitudes is not observed in the fishes of other 
families, and we have to face the problem how it is that the same 
change of conditions produces different effects in different cases. It 
is possible, however, to investigate the subject by analysis and 
experiment. The point on which I wish to insist here is that specific 
characters are intelligible when regarded as the necessary conse- 
quences of the conditions of life, while the supposition that they are 
of any use or significance in the struggle for existence is in a vast 
number of cases unsupported by any evidence. 
Having thus indicated the reasons for rejecting the conclusion 
that all distinguishing characters are adaptive or advantageous, we 
may proceed to consider the origin of adaptations. 
It may be truly said that no animal is without adaptations ; it 
must be provided with some means by which the essentials of life 
are secured, but these means may be exceedingly simple or exceed- 
ingly complex. But there is another idea implied in the conception 
or adaptation, the idea of unity in diversity, of parts essentially 
similar being modified in different animals for different purposes, 
being adapted in many cases for purposes quite different from that 
which they originally served, as in the case of the fore-leg becoming 
in birds the wing. It is possible to trace such modifications and 
more or less disguised homologies without any reference to the doc- 
trine of evolution. The principle of descent with modification gives 
the explanation of the phenomenon. The unity of plan is due to 
heredity; the divergence, to adaptation to changed conditions. But 
we must pursue the investigation further, and endeavour to discover 
how the modification is effected. It may be asked, since we admit 
that adaptation is such a prevalent phenomenon in the animal king- 
dom, even if it is not universal and exclusive, what other explana- 
tion of it is required than natural selection? In reply to this I 
would urge, in the first place, that natural selection implies and 
assumes the appearance of variations, of slight modifications by 
virtue of which certain individuals differ from their brethren and 
from their parents, in fact from all pre-existing individuals. The 
real explanation of evolution therefore lies in the explanation of in- 
dividual variations. We admit that they occur, but how and why ? 
Darwin held that the use or disuse of organs and the direct 
action of conditions caused modifications of individuals in definite 
directions, and that these modifications were hereditary in some 
degree. Now, if once we admit this, selection becomes a secondary 
and subordinate factor. For if a new set of conditions or a change 
of habits caused a hereditary change of structure in all the indi- 
viduals exposed to it, continuous modification would take place even 
if all the individuals generated survived, or if those which were 
