There
are, however, some other aspects of the question that need to be
considered, and some theoretical views which require to be carefully
examined, having done which we shall be in a condition to state the
general conclusions to which the facts and reasonings at our command
seem to point.
_Sterility due to changed Conditions and usually correlated with other
Characters, especially with Colour._
The evidence already adduced as to the extreme susceptibility of the
reproductive system, and the curious irregularity with which infertility
or sterility appears in the crosses between some varieties or species
while quite absent in those between others, seem to indicate that
sterility is a characteristic which has a constant tendency to appear,
either by itself or in correlation with other characters. It is known to
be especially liable to occur under changed conditions of life; and, as
such change is usually the starting-point and cause of the development
of new species, we have already found a reason why it should so often
appear when species become fully differentiated.
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