320 GENERAL ZOOLOGY 



plasm, the hereditary substance of the germ cells, while fluctuat- 

 ing or Darwinian variations are due to variations in the somatic 

 cells caused by environmental conditions. 



The work of De Vries gave great impetus to the search for 

 mutations and their study in animals and plants. The sport 

 variations of Darwin, which correspond to mutations, have been 

 observed in many forms, though not in great abundance, nor 

 under the conditions found in Oenothera. The mutations 

 appearing spontaneously and also produced by radiation in 

 Drosophila, black sheep in an otherwise white flock, albinos 

 (individuals lacking pigment in the skin, hair, and eyes), horn- 

 less cattle, mule-footed pigs, six-fingered hands and six-toed feet 

 in man, are all common examples of mutations. It has been 

 established that these conditions are hereditary, which means 

 that they are germinal rather than somatic in origin. Many 

 mutations are abnormal in nature, but it is possible that some 

 of the more favorable characters might have survival value and 

 serve as a basis for species change. So far as Darwin's theory 

 is concerned, the occurrence of mutations would hasten the 

 process of evolution for the reason that mutation provides abrupt 

 and quick changes in place of the slower changes resulting from 

 the accumulation of smaller fluctuating variations. 



It is rather curious that the mutations in Oenothera, discovered 

 by De Vries and others since, differ in their origin from mutations 

 in other forms. According to some investigators, Oenothera 

 lamarckiana is not a pure stock but some sort of complex 

 hybrid whose heterozygous character is shown from time to time 

 by the production of so-called mutations. It cannot be an 

 ordinary Mendelian hybrid because the mutations are not pro- 

 duced in Mendelian ratios. In some cases it has been shown 

 that mutations breed true, not because they are true (new) 

 species but because they are heterozygotes that do not produce 

 viable homozygous offspring. In another case, that of Oenothera 

 gigas, distinguished by its large size, the mutant condition is 

 accompanied by a doubling of the chromosomes. Even though 

 mutation may be an important factor in evolution, the mutations 

 of the primrose cannot be regarded as incipient species, unless, 

 as some maintain, hybridism plays a part in the evolution of 

 species. Though mutations occur in many forms in the absence 

 of any visible changes in the chromosomes, it is generally thought 



