PRESENT ASPECTS OF EVOLUTION 295 



lus of external influences, supplemented by sexual 

 reproduction. 



But the very constitution of the germ-plasm and its 

 relation to the body absolutely forbids the transmis- 

 sion of acquired somatic characteristics and of the 

 special effects of use and disuse. Muscular activity 

 promotes general health, and might thus conduce to 

 better-nourished germ-cells and to more vigorous and 

 therefore athletic descendants. The exercise of the 

 muscles might possibly cause such a condition of the 

 blood that the portion of the germ-plasm representing 

 the muscular system of the next generation might be 

 especially nourished or stimulated. Thus an athletic 

 parent might produce more athletic children. 



But let us imagine twin brothers of equal muscular 

 development. One from childhood on exercises the 

 lower half of his body ; the other, the upper. Both 

 take the same amount of exercise, and have perhaps 

 equal muscular development, but located in different 

 halves of the body. Now it is hard to conceive that it 

 can make any difference in the nourishing or stimulat- 

 ing influence of the blood, whether the muscular activ- 

 ity resides in one half of the body or the other. The 

 children might be exactly alike. 



One man drives the pen, a second plays the piano, 

 and a third wields a light hammer. All three use 

 different muscles of the hand and arm. How can this 

 use of special muscles stamp itself upon the germ-cells 

 in such a way that the offspring will have these special 

 muscles enlarged ? Granting that external influences 

 of environment and bodily condition may effect the 

 germ-cells ; granting even that some of the most gen- 

 eral effects of use and disuse might be transmitted, 



