2. A change of conditions occurs which threatens the existence of the
species, but the _two varieties_ are adapted to the changing conditions,
and, if accumulated, will form two new _species adapted to the new
conditions_.
3. Free crossing, however, renders this impossible, and so the species
is in danger of extinction.
4. If _sterility_ could be induced, then the pure races would increase
more rapidly and replace the old species.
5. It is admitted that _partial sterility_ between _varieties_ does
occasionally occur. It is admitted the _degree_ of this sterility
_varies_. Is it not probable that Natural Selection can accumulate these
variations and thus save the species?
If Natural Selection can _not_ do this, how do species ever arise,
except when a variety is isolated?
Closely allied species in distinct countries being sterile is no
difficulty, for either they diverged from a common ancestor in contact,
and Natural Selection increased the sterility, or they were isolated,
and have varied since, in which case they have been for ages influenced
by distinct conditions which may well produce sterility.
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