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.

176-198.] [Footnote 42: See Kerner's _Flowers and their Unbidden Guests_ for numerous other structures and peculiarities of plants which are shown to be adaptive and useful.] [Footnote 43: _Nature_, vol.xx.p.