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 trans- 
formed 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 geneti- 
cists find linked with so many mutating genes in the labora- 
tory 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 great- 
est handicap is the fact that it usually develops in the midst 
of a population so similar to itself that it is capable of in- 
terbreeding 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 con- 
spectus of the parental species which, in the course of con- 
siderable 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 re- 
cessive as often as they have proved in the laboratory (Mor- 
gan 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 sub- 
merged in the parental population. It becomes apparent that 
the transformation of a mutant race into a species must ordi- 
narily 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 
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