Cirripedes long appeared to me to present a case of very
great difficulty under this point of view; but I have been enabled, by
a fortunate chance, elsewhere to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, in the
case of both animals and plants, species of the same family and even
of the same genus, though agreeing closely with each other in almost
their whole organisation, yet are not rarely, some of them
hermaphrodites, and some of them unisexual. But if, in fact, all
hermaphrodites do occasionally intercross with other individuals, the
difference between hermaphrodites and unisexual species, as far as
function is concerned, becomes very small.
From these several considerations and from the many special facts
which I have collected, but which I am not here able to give, I am
strongly inclined to suspect that, both in the vegetable and animal
kingdoms, an occasional intercross with a distinct individual is a law
of nature. I am well aware that there are, on this view, many cases of
difficulty, some of which I am trying to investigate. Finally then, we
may conclude that in many organic beings, a cross between two
individuals is an obvious necessity for each birth; in many others it
occurs perhaps only at long intervals; but in none, as I suspect, can
self-fertilisation go on for perpetuity.
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