VIII A THEORY OF HEREDITY 277 



are essentially nothing more than vehicles, in which 

 are contained small groups of the physiological units 

 in a fit state for obeying their proclivity towards the 

 structural arrangement of the species they belong to. 

 ... If the likeness of offspring to parents is thus 

 determined, it becomes manifest, a jpriori, that besides 

 the transmission of generic and specific peculiarities, 

 there will be a transmission of those individual peculi- 

 arities which, arising without assignable causes, are 

 classed as ' spontaneous.' . . . 



" That changes of structure caused by changes of 

 action must also be transmitted, however obscurely, 

 from one generation to another, appears to be a de- 

 duction from first principles — or if not a specific 

 deduction, still, a general implication. . . . Bringing 

 the question to its ultimate and simplest form, we may 

 say that as on the one hand physiological units will, 

 because of their special polarities, build themselves 

 into an organism of a special structure, so on the other 

 hand, if the structure of this organism is modified by 

 modified function, it will impress some corresponding 

 modification on the structures and polarities of its 

 units. The units and the aggregate must act and 

 react on each other. The forces exercised by each 

 unit on the aggregate, and by the aggregate on each 

 unit, must ever tend towards a balance. If nothing 

 prevents, the units will mould the aggregate into a 

 form in equilibrium with their pre-existing polarities. 

 If contrariwise, the aggregate is made by incident 

 actions to take a new form, its forces must tend to 

 re-mould the units into harmony with this new form ; 



