LIMITS OF NATURAL SELECTION. 5 



of natural selection is necessary to keep animals 

 from degenerating — thus apparently implying that 

 variations are even more likely to be detrimental 

 than useful. 



In connection with this belief in the sole efficacy 

 of natural selection, the old theory of the inheritance 

 of acquired characters has been abandoned, and in 

 its place has been substituted Professor Weismann's 

 theory of heredity, according to which the inheri- 

 tance of acquired characters is absolutely impossible. 

 This theory may be briefly stated as follows : In 

 the germ-cells of animals there is a certain amount of 

 "germ-plasm," a substance of very complex physico- 

 chemical composition, of which one part gives rise 

 to the development of the incipient animal and is 

 thereby used up ; but the other part remains in 

 the body of the developing animal, unchanged in 

 composition but increasing in quantity. This un- 

 changed part then forms the germ-plasm from which 

 the next succeeding generation develops and receives 

 its share of the unchanged germ-plasm. The same 

 process is repeated in all generations, one part of 

 the germ-plasm being used up to produce the bodies 

 of each generation, the other part of the germ- 

 plasm remaining forever unchanged. As all the 

 generations arise from the same germ-plasm, they 

 are therefore alike, with this exception (which is 



