Origin of Species Variorum

it would clearly be advantageous to two varieties or incipient species, if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate. In the first place, it may be remarked that distinct regions are often inhabited by groups of species and by single species which when brought together and crossed are found to be more or less sterile; now it could clearly have been of no advantage to such separated species to have been rendered mutually sterile, and consequently this could not have been effected through natural selection; but it may perhaps be argued with truth, that, if a species were rendered sterile with some one compatriot, sterility with other species would probably follow as a necessary contingency. In the second place, it is as much opposed to the theory of natural selection as to that of special creation, that in reciprocal crosses the male element of one a second form, whilst at the same time the male element of this second form is enabled freely to fertilise the first form. ↑

But in considering the probability of natural selection having come into action, one great difficulty will be found to lie in the existence of many graduated steps from very slightly lessened fertility to utter and absolute sterility. It may be admitted, on the principle above explained, that it would profit an incipient species if it were rendered in some slight degree sterile when crossed with its parent-form or with some other variety; for thus fewer bastardised and deteriorated offspring would be produced to commingle their blood with the newly-forming variety. But he who will take the trouble to reflect on the steps by which this first degree of sterility could be increased through natural selection