134 HEREDITY AND SELECTION IN SOCIOLOGY 



modification of the shells of the Celebes land-snails cannot be 

 due to natural selection ; but Weismann has pointed out that 

 the shell is adapted to cover the viscera of the animal ; and that, 

 as we know neither the kind of food peculiar to the different 

 species of snails, nor the varying nutritive value of these different 

 kinds of food, nor the changes in secretion, assimilation, and 

 excretion consequent on these differences of nutritive value, 

 it is impossible to state precisely in what way the digestive 

 apparatus of each species is adapted to its mode of life, nor to 

 appreciate the value of the differences that obtain, nor to 

 establish even an approximate correlation between the digestive 

 apparatus of each snail and its food.^ But recent researches 

 have shown that changes of nourishment may, as a matter of 

 fact, produce various modifications of the digestive tract, and 

 consequently of the shell which covers it ; and the latter, in adapt- 

 ing itself to the snaU's conditions of life, comes within the scope 

 of natural selection. 



We have said that the origin of every variation is to be found 

 within the germ-plasm. It is through perturbations of intra- 

 germinal equilibrium that variations arise, and thus these 

 intragerminal variations constitute the material on which 

 natural selection, in Darwin's sense of the term, can operate. 

 But numerous variations do not attain biological value, and are 

 consequently beneath the reach of natural selection. Never- 

 theless, such variations may become indices of a species, mani- 

 festing themselves as " indifferent " characters, although it is 

 not these characters which determine the constitution of a species. 

 They may be compared with the moss in a forest, which latter 

 is characterised by its trees ; the moss might be entirely extir- 

 pated without affecting the character of the forest. The 

 " organisation of the organism," the harmony of its different 

 parts, the correlated variations which ensue from a change in 

 many given parts, are due, not to natural selection as Darwin 

 ^ Weismann, Vortrdge ilber Deszendenztheorie, ii. 253. 



