202 THE THEORIES OF EVOLUTION 



acters. The gathering of negative evidence is no easy 

 matter. 



Spencer, who always held that the heredity of ac- 

 quired characters was as incontestable as the heredi- 

 tary transmission of any other racial or familial 

 character, explains in one of his earliest works, his 

 "Principles of Biology" (1864) , that owing, not to the 

 nature of the discussion but to the nature of the 

 phenomena themselves, scanty evidence can be fur- 

 nished of hereditary transmission. 



"Changes produced in the sizes of parts by changes 

 in their amount of action, are mostly unobtrusive. A 

 muscle which has increased in bulk, unless the altera- 

 tion is extreme, passes without remark. Such nerv- 

 ous developments as are possible in the course of a 

 single life, cannot be seen externally. And if the 

 changes of structure worked in individuals by changes 

 in their habits are thus difficult to trace, still more 

 difficult to trace must be the transmission of them: 

 further hidden, as this is, by the influences of other in- 

 dividuals who are often otherwise modified by other 

 habits, or may be due to natural or artificial selec- 

 tion." 6 The heredity of acquired characters is to 

 Spencer what the contrary thesis is to Weismann — a 

 theoretical and logical postulate. 



These two remarkable men entered into a very 



s Principles of Biology, Vol. I, p. 307. 



