The cowslip and primrose are moderately
sterile, yet occasionally produce hybrids: now these hybrids, two or
three or a dozen in a whole parish, occupy ground which _might_ have
been occupied by either pure species, and no doubt the latter suffer to
this small extent. But can you conceive that any individual plants of
the primrose and cowslip, which happened to be mutually rather more
sterile (i.e. which when crossed yielded a few less seeds) than usual,
would profit to such a degree as to increase in number to the ultimate
exclusion of the present primrose and cowslip? I cannot.
My son, I am sorry to say, cannot see the full force of your rejoinder
in regard to the second head of continually augmented sterility. You
speak in this rejoinder, and in par. 5, of all the individuals becoming
in some slight degree sterile in certain districts; if you were to admit
that by continued exposure to these same conditions the sterility would
inevitably increase, there would be no need of Natural Selection. But I
suspect that the sterility is not caused so much by any particular
conditions, as by long habituation to conditions of any kind.
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