MECHANISM AND HEREDITY 55 



nothing more than a collection of material of a 

 certain composition, and capable, in a suitable 

 environment, of indefinite quantitative in 

 crease or growth. Parent and offspring were 

 supposed to be similar because they had both 

 sprung from germ-plasm of the same com 

 position. This germ-plasm could be added 

 to, divided, or mixed with other germ-plasm 

 in sexual reproduction, just as if it were so 

 much treacle. 



It seems perfectly clear that germ-plasm of 

 so simple a character as this could by itself 

 furnish no explanation whatever of the de 

 velopment from it of the adult organism with 

 all its enormous complication and absolute 

 definiteness of structure. If the germ-plasm 

 were so simple, the complication and definite- 

 ness would have to be attributed to its 

 environment : whereas all the evidence points 

 to the nuclear germ-plasm as the essential 

 carrier of hereditary characters. We are thus 

 compelled, on the mechanistic hypothesis, to 

 attribute to the germ-plasm, or germinal 

 nuclear substance, a structure so arranged 

 that in presence of suitable pabulum and 

 stimuli it produces the whole of the vast and 



