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change had been brought about by Natural Selection, the 
process would have been quite different. The animals 
would have changed their habitat, and found themselves 
in a realm of darkness. Then the presence of an organ 
which was useless would be prejudicial ; and those which 
could see least would flourish most, until the blindness 
which was so advantageous as to make the difference 
between life and death would be fixed in the race. 
It is surely evident that neither of these explanations 
is, on the face of it, so probable as the explanation 
of Mr. Darwin, who believed that the unused organ tends 
to become atrophied, and that such tendency is inherited. 
But what I am now concerned to show is that Dr. Ray 
Lankester’s explanation is a process of separation which 
is not Natural Selection, and that the process which he 
describes appears to be the most improbable. 
But all these difficulties disappear if it is not necessary 
for us to exclude the occasional influence of changed 
conditions. If variation takes place before isolation, isola 
tion will prevent the variants from interbreeding with the 
rest of the species. If certain individuals of a species are 
isolated before variation begins, and the new conditions 
modify all the individuals brought under the influence, 
the isolation will protect them from intermixture with 
others which are not thus modified. In that case, we 
say with respect to such a barrier what Shakespeare says 
of our sea-girt island :— 
“ this scepter’d isle, 
This fortress, built by nature for herself 
Against infection, .... 
this little world ; 
This precious stone set in the silver sea, 
Which serves it in the office of a wall, 
Or as a moat defensive to a house.” 
—(Richard II. Act sc. i.)