MUTATIONS
The most brilliant contribution to the species problem has been the outcome of transferring the search for the cause of variation into an investigation of the factors responsible for the uniformity of individuals of successive generations. Out of this development of modern genetics has come not only a localization in the reproductive cells of the mechanism by which hereditary similarities are achieved, but an assurance that the inception of new species must take place in those same genes and a considerable acquaintance with the potentialities of genes.
These laboratory studies have always led to the conclusion that changes in genes occur in sudden jumps, sometimes of small degree, sometimes of considerable size, but always as mutations which are complete as soon as they have occurred. This concept has been held in contrast with the neo-Darwinian conception of “fluctuating variations” which, by being accumulated over many generations and bent in a given direction by the force of natural selection, would gradually give rise to the characters of new species. Under this latter interpretation there may be incipient species of the sort conceived by many taxonomists in their definitions of varieties and subspecies; under the genetic interpretation the first mutant individual embodies all that the new species will contain, and is the new species as soon as it is given an opportunity to perpetuate its mutant characters thru a population of individuals.
The genetics data are conclusive as to the frequent occurrence of mutations in the laboratory; there are numerous records of the appearance of similarly mutant individuals in nature, but there is little satisfactory evidence that these are the individuals out of which new species are made. Perhaps the best body of proof is that of Crampton's (1909-1928) on the development of geographically isolated races of snails from mutant stocks in various Pacific Islands. In the taxonomic literature there are a few other suggestions of similarly mutant origins of existent species, but such an experienced field worker as Chapman is quoted (H. F. Osborn, 1926) as finding among the birds next to no species which might be interpreted as owing their origin to mutation.
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