e. produce half the full number of offspring. Now try
and make (by Natural Selection) A and B absolutely sterile when crossed,
and you will find how difficult it is. I grant, indeed it is certain,
that the degree of sterility of the individuals of A and B will vary,
but any such extra-sterile individuals of, we will say, A, if they
should hereafter breed with other individuals of A, will bequeath no
advantage to their progeny, by which these families will tend to
increase in number over other families of A, which are not more sterile
when crossed with B. But I do not know that I have made this any clearer
than in the chapter in my book. It is a most difficult bit of reasoning,
which I have gone over and over again on paper with diagrams.
I shall be intensely curious to see your article in the _Journal of
Travel_.
Many thanks for such answers as you could give. From what you say I
should have inferred that birds of paradise were probably polygamous.
But after all, perhaps it is not so important as I thought. I have been
going through the whole animal kingdom in reference to sexual selection,
and I have just got to the beginning of Lepidoptera, i.
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