rentiation of
species, because the same result is brought about by the incipient
species acquiring different habits or frequenting a different station;
and also by the fact that different varieties of the same species are
known to prefer to pair with their like, and thus to bring about a
physiological isolation of the most effective kind. This part of the
subject will be again referred to when the very difficult problems
presented by hybridity are discussed.[50]

_Cases in which Isolation is Ineffective._

One objection to the views of those who, like Mr. Gulick, believe
isolation itself to be a cause of modification of species deserves
attention, namely, the entire absence of change where, if this were a
_vera causa_, we should expect to find it. 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. 603.]

[Footnote 44: _Nature_, vol. xxxviii. p. 328.]

[Footnote 45: A very remarkable illustration of function in an
apparently useless ornament is given by Semper. He says, "It is known
that the skin of reptiles encloses the body with scales. These scales
are distinguished by very various sculpturings, highly characteristic of
the different species. Irrespective of their systematic signifi