377
rier whatever would be needed. The only condition
under which such a phenomenon is possible would
be in connection with migration from one district to
another. But in this case we must suppose that the
advanced guard always consists of those whose similar
variations will be favourable to the transmutation of
species, and one does not see exactly why this should
be so. Again, the separating barrier must be sufficient
to secure isolation for breeding purposes ; but if so, it is
probable that the circumstances of the new habitat will
be different, and it is difficult to believe that these different
conditions will not produce some direct effect upon the
organism. How are we, in such cases, to tell exactly
which modifications are due to sexual reproduction, pure
and simple, and which to the effect of external conditions ?
Dr. Weismann recognises this difficulty, when he says:—
“ In single cases it may be difficult, or for the present impossible,
to decide whether we have before us a climatic form or a local form
arising from other causes.”—(Studies in the Theory of Descent, vol.
P■ 4^-)
As an illustration of this difficulty, we may refer to
Dr. E. Ray Lankester’s explanation of the occurrence of
blind animals in caves and in the deep sea. He says :—
“ But this instance is really fully explained by the theory of
Natural Selection acting on congenital fortuitous variations. Many
animals are thus born with distorted or defective eyes whose parents
have not had their eyes submitted to any peculiar conditions.
Supposing a number of some species of Arthropod, or Fish, to be
swept into a cavern, or to be carried from less to greater depths
in the sea,—those individuals with perfect eyes would follow the
glimmer of light and eventually escape to the outer air or the
shallower depths, leaving behind those with imperfect eyes to breed
in the dark place. A natural selection would thus be effected.
In every succeeding generation this would be the case, and even
those with weak but still seeing eyes would in the course of time
escape, until only a pure race of eyeless or blind animals would
be left in the cavern or deep sea.”—(.Encyclopaedia Britannica.
vol. xxiv., p. 8IQ.)