Once a species becomes mixed, it will be seen from the foregoing that the genes themselves never blend, and no blending of species can take place, other than superficially by the dominance of certain genetic qualities, or materially, by the elimination of all individuals (possibly over a period of many generations) who may exhibit variant characteristics. Actually this latter process can scarcely be called a blend, since it implies the elimination of a specific part of the combined inheritance.
Amongst the lower animals, where the genetic pattern is simple, it is relatively easier, of course, to eliminate or isolate the individuals with mixed genes, and to breed only from those which represent one or other pure types. By such a careful eugenic process the Irish Wolfhound was actually bred back into existence after it had been extinct as a pure breed for several generations; but where mankind is concerned, the complexity is such that there is little possibility of such action, although it is possible to go far with the aid of the dominant-recessive pattern in this direction.
Theoretically, in fact, if a Hottentot were crossed with an Eskimo, pure Hottentot and pure Eskimo types could be produced from amongst the offspring, but the number of genetic variables in human beings is so great that the number of offspring required to produce, according to the laws of mathematics, one pure specimen of each sub-species would run into millions.
This is a passage from the second chapter of Eugenics and Race, a booklet now available from Daybreak Publications (here):