54 GENETICS AND EUGENICS 



inherited variations similar to those which the environment 

 directly produces in the body, he invented the hypothesis of 

 parallel modification of germ-plasm and soma, to which refer- 

 ence has already been made. 2. To account for the appar- 

 ent inheritance of the effects of use and disuse, he invented 

 the hypothesis of germinal selection. On this view the various 

 determiners which compose the germ-plasm are competing 

 with each other in a struggle for nourishment, just as animals 

 and plants struggle with each other for existence in the world 

 at large. Sometimes one determiner gets more nourishment, 

 sometimes another; but whichever one gets most nourish- 

 ment, grows largest, and would consequently give rise to a 

 plus variation of a corresponding part or organ of the body. 

 When one determiner gets more nourishment, that is, pro- 

 duces a plus variation, some other determiner gets less and 

 so produces a minus variation. Thus there is perpetual varia- 

 tion in the parts and organs of the body, which affords 

 abundant material for natural selection to act upon. For if 

 any essential organ gets too small, its possessor is eliminated. 

 But if the organ which undergoes minus variation is a use- 

 less one, no disadvantage results to the organism; on the 

 contrary, there is more nourishment left for essential organs, 

 which therefore grow at the expense of the useless ones. 

 Thus through natural selection useless organs tend to dimin- 

 ish and ultimately to disappear altogether, while essential 

 organs (those most used) grow in size and activity. An 

 apparent inheritance of the effects of use and disuse results. 

 Modern research supports Weismann's theory of nuclear 

 determiners to this extent. It appears highly probable that 

 special chemical substances necessary for the production of 

 particular variations are located in particular parts of the 

 cell, possibly in chromosomes. It is also conceivable that 

 these substances may vary from cell to cell in amount or 

 quality, and that under a constant environment variation in 

 particular organs affected may thus result. But it is not neces- 

 sary to suppose, as Weismann did, that these groups of sub- 

 stances are engaged in a struggle of any sort, with each other. 



