136 HEREDITY AND SELECTION IN SOCIOLOGY 



sufficiently great, it follows ttat a large number of different 

 variations, either towards plus or towards minus, must set in. 

 Those variations whicli are favourable, wMch serve to adapt 

 the individuals possessing them to the changed conditions of life, 

 will be favoured by natural selection, and the others rejected ; 

 that is to say, the individuals which possess them will not sur- 

 vive. Thus, practically every time a new adaptation is necessary, 

 natural selection has the requisite material to choose from. 



On the one hand, therefore, the theory of Nageli is justified, in 

 so far as every organic variation has its origin in the germ-plasm, 

 and is due to germinal selection ; and, on the other hand, NageH's 

 theory must be completed by that of Darwin, for characters 

 which are of biological importance to the species must be selected 

 according to the law of the survival of the fittest. Germinal 

 selection by itself is capable of explaining only the secondary 

 variations ; natural selection alone is too dependent on chance 

 for the adaptation of the species to be left solely to it. The 

 evolution of a species is best interpreted as a combination of, 

 or a co-operation between, germinal and natural selection. 

 And thus the seemingly contradictory doctrines of Nageli and 

 Darwin are reconciled. 



B. — Influence op the Environment on Species. 



We have seen that secondary variations may be brought out, 

 either in the species as a whole or in certain individuals of a 

 species, which do not attain biological value. The maintenance 

 of such secondary variations depends, in great part, on the 

 chances of amphimixis ; for should the determinants of a secon- 

 dary variation a in an individual A be able to combine with the 

 homologous determinants of an identical variation in an indi- 

 vidual B, the variation will be preserved ; and its maintenance 

 will continue as long as the chances of amphimixis are favourable 

 to it. If the secondary variation thus produced be in correlative 



