360 ACTION OF NATURAL SELECTION 



lesser degree upon the " determinants ' representing 

 them in the germ-plasm. 



It should be mentioned that the influence of somatic 

 variations on the germ-plasm through the agency of 

 various secretions has already been suggested by De- 

 lage.* Though he does not admit Weismann's doctrine 

 of determinants, he thinks that the ovum may contain 

 specific substances of an identical nature to those con- 

 tained in the cells of the principal classes of tissues, 

 such as the nervous, muscular, and perhaps glandular. 

 Conditions of life such as climate and food, which 

 through the intermediation of the blood influence the 

 constituents of certain of the bodv tissue cells, will 



t/ 7 



therefore influence the same substance in the ovum, or 

 produce hereditary variations. 



The hypothesis of specific secretions is of distinct 

 help in accounting for certain apparent instances of 

 the inherited effects of use and disuse. As we have 

 seen in a former chapter, Darwin found that the rela- 

 tive size of the brain of the domestic rabbit has con- 

 siderably diminished. Possibly this may have been 

 the result of more ample food, and of artificial selection 

 of individuals with large bodies and small heads, and 

 of panmixia (cessation of Natural Selection), but it 

 seems almost more probable that it is due, at least in 

 part, to the inherited effects of disuse. Thus a rabbit, 

 when kept in captivity, would need to use its brain but 

 little, and hence the excretions and secretions of the 

 nervous tissues would be diminished. The " determi- 

 nants ' in the germ-plasm corresponding to these 

 would be less stimulated than in wild rabbits, and 

 * " Heredite," pp. 806 to 812, Paris, 1895. 



