THE ORIGIN OF VARIATIONS 233 



(amphimixis) the traits of two lines of ancestry, and with the 

 union a possibility of different combinations in the segregation 

 of characteristics. Or by such union, recessive characteristics 

 may be brought out, leading to divergent types in the race. 

 This principle, advocated by Weismann, leaves unexplained 

 the origin of the factors in the germ plasm, but interprets the 

 changes that may arise, as due to shuffling about of the 

 characters already present. Other biologists interpret am- 

 phimixis as bringing about the exactly opposite result, viz., 

 keeping the race true to type, and preventing variations. 



Mutations. A number of biologists believe that new types 

 arise suddenly, by jumps or mutations, which first appear as 

 freaks of nature or "sports." The botanist de Vries discovered 

 a variety of primrose which underwent a spontaneous change of 

 type, sufficiently well marked to make of it a new variety, if not 

 a new species. It bred true to its type, and showed no tendency 

 to revert to the ancestral form. De Vries concluded, and many 

 biologists agree with him, that freaks or sports appear infre- 

 quently in the history of every species, and serve as centers of 

 departure from old types. Such mutations were known to 

 Darwin and the earlier evolutionists, the race of Ancon sheep 

 being an historic example. 



Mutations may be due to the chance union of recessive char- 

 acteristics, which, as in Prof. Morgan's flies, would be lost again 

 by promiscuous or indiscriminate breeding. Prof. Tower has 

 been able to breed in the laboratory distinct types of the potato 

 beetle, which differ markedly from the ancestral type from 

 which they sprang, and he has found the same distinct types 

 existing wild in nature and regarded as different species. Here 

 an experiment was performed in the laboratory, which had 

 been done on a larger scale in nature, with the advantage in the 

 laboratory, because the starting point, a mutant, was known. 

 But again the result may be interpreted as due to shifting of 

 germinal characteristics, followed by discriminating breeding. 



If, in any of these cases, the variation is useful to the organism 

 in its struggle for existence, the chances of living and of mating 

 are increased, which would result in the numerical increase of 



