There
are, it must be admitted, cases of special difficulty opposed to the theory
of natural selection; and one of the most curious of these is the existence
in the same community of two or three defined castes of workers or sterile
female ants; but I have attempted to show how these difficulties can be
mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the ninth chapter, which
seem to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two distinct kinds of trees to be
grafted together; but that it is incidental on differences confined to the
reproductive systems of the intercrossed species. We see the truth of this
conclusion in the vast difference in the results of crossing the same two
species reciprocally--that is, when one species is first used as the father
and then as the mother. Analogy from the consideration of dimorphic and
trimorphic plants clearly leads to the same conclusion, for when the forms
are illegitimately united, they yield few or no seed, and their offspring
are more or less sterile; and these forms belong to the same undoubted
species, and differ from each other in no respect except in their
reproductive organs and functions.
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