I] THEORIES OF HEREDITY 143 



fertilised egg, and thus arises the variation between 

 separate individuals. Further, although by his theory 

 changes brought about in the body-plasm cannot be 

 transferred to the germ-plasm, yet influences acting 

 on the germ-plasm itself may modify it and so their 

 effects will be transmitted. The most important of 

 these influences is nourishment, which may favour 

 some units of the germ-plasm rather than others. 

 He further supposes that there may be competition 

 for nourishment among the different units (' deter- 

 minants') so that some increase at the expense of 

 others, and if this process should be continued 

 through a series of generations, certain characters 

 would show a steady increase while others corre- 

 spondingly decrease. Variation thus arises by changes 

 brought about in the germ-plasm, and by the 

 recombination of varied ancestral germ-plasms in 

 each generation. Such variations will be inherited, 

 and in this respect will differ entirely from changes 

 brought about in the body during its life by the 

 action of environment. 



It has been shown that in the earlier theories 

 of heredity it was assumed that the germ-cells were 

 produced by the body, and that they must therefore 

 be supposed either to contain samples of all parts 

 of it, or at least some kind of units derived from 

 those parts and able to cause their development 

 in the next generation. Gradually, as the study 



