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.)