EVOLUTION 509 



If such modifications are acquired by both sexes they 

 are transmitted by heredity from generation to generation. 



One of the weaknesses in Lamarck's hypothesis appears 

 in his illustration of the snake. If we should grant that 

 inheritance of the effects of disuse of the legs might possi- 

 bly explain their absence in snakes, still it would not ex- 

 plain the origin of the snake's deiire to glide. That is, of 

 course, as much a characteristic of the snake as the absence 

 of legs. 



Other arguments against the validity of Lamarckisiri 

 are : first, that no one has ever been able to prove, by ex- 

 periment or otherwise, that the effects of use (the so-called 

 " acquired characters ") are inheritable, while irinumerable 

 facts indicate that they are not; ^ second, the hypothesis 

 could apply only to the animal kingdom, since plants in 

 general have no nervous and muscular activities like those 

 of animals. A hypothesis of organic evolution, to be valid, 

 must apply equally to both plants ^nd animals. 



^.Darwin's Hypothesis. — This will be outlined in the 

 next chapter. 



'' -■ ; 



/For exapiple, sons of blacksmiths do not, have unusually strong arms; 

 children of pianistsseldom, if ever, manifest any unusual manual dexterity 

 at' the keyboard. ■■' ■ 



