The same thing may be shown in this way. It is perfectly obvious that where the improvement of type in a linear series is concerned (monotypic evolution), free intercrossing, far from being a hindrance to the process, _is the very means by which the process is accomplished_. Improvement here ascends by successive steps, in successive generations, simply _because_ of the general intercrossing of the generally most fit with the result that the species, _as a whole_, gradually becomes transformed into another species, _as a whole_. Therefore, it would be mere fatuity in any one to adduce free intercrossing as a "difficulty" against natural selection alone being competent to produce evolution of this kind. But where the kind of evolution is that whereby the species is _differentiated_--where it is required, for instance, to produce different structures in different portions of the species, such as the commencement of a fighting spur on the wing of a duck, or _novel_ characters of any sort in different groups of the species--free intercrossing is no longer a condition to, but an absolute preventive of, the process; and, therefore, unless checked as between each portion of the species by some form of homogamy other than natural selection, it must effectually inhibit any _segregation_ of specific types, or divergence of character.
Hence it is that, while no Darwinian has ever questioned the power of unaided selection to cause _improvement of character in successive generations_, in common now with not a few other Darwinians I have emphatically denied so much as the abstract possibility of selection alone causing a _divergence of character in two or more simultaneous lines of change_.
And, although these opposite views cannot be reconciled, I am under the impression that they do admit of being explained. For I take them to indicate a continued failure to perceive the all-important distinction between evolution as monotypic and polytypic. Unless one has fully grasped this distinction, and constantly holds it in mind, he is not in a position to understand the "difficulty" in question; nor can he avoid playing fast and loose with natural selection as possibly the sole cause of evolution, and as necessarily requiring the co-operation of some other cause. But if he once clearly perceives that "evolution" is a logical genus, of which the monotypic and the polytypic forms are species, he will immediately escape from his confusion, and find that while the monotypic form may be caused by natural selection alone the polytypic form can never be so caused.