278 A THEORY OF HEREDITY VIII 



and to say that the physiological units are in any 

 degree so re-moulded as to bring their polar forces 

 towards equilibrium with the forces of the modified 

 aggregate, is to say that when separated in the shape 

 of reproductive centres, these units will tend to build 

 themselves up into an aggregate modified in the same 

 direction." (P. 256.) 



Thus, then, Mr. Herbert Spencer definitely assumes 

 an order of molecules or units of protoplasm — lower 

 in degree than the visible cell-units or plastids — to 

 the '' polar forces " of which and their modification by 

 external agencies and interaction, he ascribes the ulti- 

 mate responsibility in reproduction, heredity, and 

 adaptation. 



I am unable to say whether Mr. Darwin was ac- 

 quainted with or had considered Mr. Herbert Spencer's 

 hypothesis of physiological units, when in 1868 he 

 published his own provisional hypothesis of Pangenesis. 

 But an examination of the bearings of the two hypo- 

 theses shows that the former does not render the latter 

 superfluous, nor is the one inconsistent with the other. 

 Mr. Darwin wished to picture to himself and to enable 

 others to picture to themselves a process which would 

 account for (that is, hold together and explain) not 

 merely the simpler facts of hereditary transmission, 

 but those very curious though abundant cases in which 

 a character is transmitted in a latent form and at last 

 reappears after many generations, such cases being 

 known as " atavism " or " reversion " ; and again those 

 cases of latent transmission in which characteristics 

 special to the male are transmitted to the male off- 



