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In Ireland we have an
excellent test case, for we know that it has been separated from Britain
since the end of the glacial epoch, certainly many thousand years. Yet
hardly one of its mammals, reptiles, or land molluscs has undergone the
slightest change, even although there is certainly a distinct difference
in the environment both inorganic and organic. That changes have not
occurred through natural selection, is perhaps due to the less severe
struggle for existence owing to the smaller number of competing species;
but, if isolation itself were an efficient cause, acting continuously
and cumulatively, it is incredible that a decided change should not have
been produced in thousands of years. That no such change has occurred in
this, and many other cases of isolation, seems to prove that it is not
in itself a cause of modification.
There yet remain a number of difficulties and objections relating to the
question of hybridity, which are so important as to require a separate
chapter for their adequate discussion.
FOOTNOTES:
[Footnote 41: See _Origin of Species_, pp.


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