404 EVOLUTION AND GENETICS 



Objection to the mutation theory as an explanation of species- 

 formation has been based chiefly on the repeated assertion that 

 lamarckiana was probably a hybrid, and that the mutants were 

 merely different strains separated from the hybrid. Such a view 

 is plausible, for a homozygous recessive based on two determiners 

 would have only one chance in sixteen of appearing, on three 

 determiners only one chance in sixty-four, etc. However, a 

 recessive once produced would be unable to produce the charac- 

 ters of its parents among its offspring, as was the case with 

 scintillans. 



Examples of Mutants. Fortunately, although species very 

 productive of mutants are open to interpretation as hybrids, there 

 are many examples of apparently stable mutants. Not the least 

 of these are the classic examples of the Ancon sheep and polled 

 Hereford cattle. Each is said to have originated from a single 

 mutant. The sheep first appeared in the flock of Seth Wright, a 

 Massachusetts farmer, in 1791, and the hornless Hereford mutant 

 in a herd of horned Herefords at Atchison, Kansas, in 1889. The 

 fact that the mutant character proved to be dominant over the 

 normal condition in each case is another argument against the 

 hybrid-species interpretation, since dominant genes, if present, 

 are expressed. If separated as homozygous dominants from a 

 hybrid race there is no somatic difference from the parent unless 

 the hybrid character is a blending or mosaic. 



In the vast amount of work done within recent years on the 

 fruit flies of the genus Drosophila many mutations have been 

 discovered which illustrate the possil^ility of very slight departure 

 of a mutant from the characters of its parents. Such slight depar- 

 tures as have been recorded in these flies have not been interpreted 

 as specific differences, in fact, in any case. Even a slight mutation, 

 however, is inherited, and if of any use to the individual or indi- 

 viduals in which it appears, may well be a deciding factor in the 

 preservation of its possessors. 



Mutations in Natural Selection. It is the operation of natural 

 selection upon such definite even though slight departures from 

 the parental form, that seems most likely to account for the 

 formation of species according to Darwin's ideas, rather than its 

 operation upon fluctuating variations. If the short-legged Ancon 

 sheep, for example, had occurred in a natural state and not under 

 domestication, and if it had depended upon speed for safety, it 



