THE ISOLATION OF SPECIES
It now remains to show how such mutant races as we have noted in the preceding sections of this study may be transformed into such large and relatively uniform populations as satisfy our concepts of species.
A mutant individual is still far from constituting a species. Its survival depends in the first place upon the condition that the new, mutant characters shall not interfere with the health of the organism. To the lethal characters which the geneticists find linked with so many mutating genes in the laboratory there are added many other controlling conditions which would kill out a large proportion of our laboratory mutations if they were exposed to the rigors of existence in nature. This negative application of the theory of natural selection would seem axiomatic—tho I must reassert, along with Crampton (1928) and others, that this is the chief aspect of the Darwinian hypothesis which seems necessary to explain species as we have met them.
But allowing that a mutant is capable of existence, its greatest handicap is the fact that it usually develops in the midst of a population so similar to itself that it is capable of interbreeding and will interbreed with this parental stock. If the mutant characters involve only a single pair of genes and if they are dominant, they will gradually disturb the conspectus of the parental species which, in the course of considerable time, should thus become a new species in the territory formerly occupied by the parental stock. But, on the other hand, mutant characters in nature are probably recessive as often as they have proved in the laboratory (Morgan 1928:59-71), and there is increasing evidence that many characters involve multiple factors in heredity. In these events, the mutant has only a remote mathematic possibility of modifying the general aspect of the parental species, and in my judgment there is every probability that it will be submerged in the parental population. It becomes apparent that the transformation of a mutant race into a species must ordinarily depend upon some sort of isolating factor which will prevent its interbreeding with closely related stocks.
Now, this isolation of species, which we may postulate, is precisely the condition which we find among the most closely
