Recapitulation and Conclusion
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A double and parallel series of facts seems to throw much light on the sterility of species, when first crossed, and of their hybrid offspring. On the one side, there is good reason to believe that slight changes in the conditions of life give vigour and fertility to all organic beings. Turning to geographical distribution, the difficulties encountered on the theory of descent with modification are serious enough. Several eminent naturalists have of late published their belief that a multitude of reputed species in each genus are not real species; but that other species are real, that is, have been independently created. This seems to the author a strange conclusion to arrive at. There is no reason why the principles which have acted so efficiently under domestication should not have acted under nature. In the survival of favoured individuals and races, during the constantly-recurrent struggle for existence, one sees a powerful and ever-acting form of Selection.