THE PRESENT POSITION OF THE THEORY OF ORGANIC EVOLUTION. 101 



definite character which is wanting in the other. The race 

 characterized by the presence is the dominant one. The best 

 instance of this is the ordinary white mouse. This when 

 crossed with the grey wild mouse yields grey offspring. Now 

 the whiteness or albinism is due to a lack of something in the 

 constitution necessary for the production of colour. Hence 

 whiteness is recessive. Such characters in the offspring are 

 said to be due to " factors " in the germ. The enthusiastic 

 supporters of these views go so far as to deny that any 

 " variations " except those of the marked character due to the 

 presence or absence of a " factor" are inheritable at all. The 

 variations in degree, such as were measured by Galton and 

 Weldon, are termed by them " fluctuations," and are declared to 

 be non-inheritable. Fluctuations are ascribed to differences 

 in the nutrition of various germs, not to differences in their 

 inherent hereditary potentiality. A difference in the hereditary 

 potentiality such as would give rise to a new race is termed 

 a " mutation," and most Mendelians are prepared to admit that 

 such mutations occasionally take place, though how or why 

 they are unable to say. A celebrated Dutch botanist, De 

 Tries, believed that he had discovered a plant {Oenothera 

 lamarckiana), the evening primrose, in the act of giving 

 off mutations ; but as this plant is of hybrid and American 

 origin, many biologists suspect that perhaps the apparent origin 

 of mutations may be only the segregation out of the characters 

 of the two parent species and the recombination of these in 

 different groupings, just as we have seen that round green peas 

 may be produced by the combination of round yellow and green 

 wrinkled peas. 



If, however, it is to be admitted, as few reasonable Mendelians 

 would deny, that our domestic breeds have been derived from 

 wild species by the appearance of inheritable mutations, then 

 it is of great interest to know more about these mutations. It 

 appears that they are nearly all due to the absence of a factor 

 which was present in the original wild species. To give a 

 familiar example: domesticated black, yellow, chocolate, and white 

 mice are known ; the wild mouse is of a grey colour technically 

 called agouti. Now this agouti when closely examined is found 

 to contain as factors, black, yellow, and chocolate, and of course 

 the wild mouse has in its constitution chromogen, the factor 

 which permits the development of colour, which the white 

 mouse entirely lacks. So that the only evolution of which the 

 more extreme Mendelians will admit the evidence is evolution 

 backwards. It has been even hinted that the primordial germ 



