THE CAUSE OF VARIATION 95 



passage Weismann says that the "ids" (hypothetical con- 

 stituents of the germ-cell, each of which is supposed to be 

 a "biological unit" virtually containing all the parts of an 

 individual) "differ very little within the same germ-plasm . . . 

 but are only absolutely alike in the case of two ids which have 

 been formed by the division of a mother-cell." To suppose that 

 any two such bodies can be absolutely alike is to run counter 

 to all we know of nature. If not even two atoms of an element 

 are exactly alike, such likeness cannot be postulated between 

 two relatively enormous bodies, each virtually, or let us rather 

 say potentially, containing all the parts of one of the higher 

 animals. 



