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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.)