146 ON VARIABILITY AND ADAPTATION 



Indeed, I am prepared to go further and to state that the idio- 

 plasm possessing its full complement of side-chains must be 

 regarded as ipso facto incapable of initiating cell multiplication. 



I base this statement upon the fact, to which I have more 

 than once called attention during the last few months, 1 that 

 it is only cells which are undifferentiated, or which have reverted 

 to a simpler, less differentiated form, that are found to undergo 

 division. Highly differentiated cells never multiply as such. 



Inheritance in Unicellular Forms. — If the primitive uni- 

 cellular individual formed by this controlling idioplasm divides 

 into two to form two individuals, each half will contain a portion 

 of the idioplasm, and if the circumstances affecting the mole- 

 cules of the new generation are the same as those affecting the 

 parent forms, then this idioplasm will continue to assimilate 

 to itself non-living matter and to endow it with its constitution 

 and properties, and the individuals controlled by this idioplasm 

 will correspond to the parent form and to each other. 



But if the circumstances affecting the filial idioplasm vary 

 from those affecting the parental, then these more unstable and 

 loosely connected side-chains will be the first to be influenced. 

 The very act of assimilation (the surrounding medium varying 

 in its composition) may lead to the substitution of other side- 

 chains, to slight variation in the composition of the idioplasm. 

 And the cell or individual developed or controlled by the idio- 

 plasm will therefore vary from the parent form, while the pro- 

 ducts of division of this second generation, containing as they 

 do this modified idioplasm, will exhibit like structural modifica- 

 tions. Here we have the simplest example of the inheritance of 

 acquired characters. 



There is, however, yet another property which we have to 

 assume. It may be laid down as one of the great laws of biology 

 that characters which are of the most recent acquirement are 

 those which are most unstable, and are first lost ; those which 

 are oldest are the last to disappear. In man, for example, the 

 first signs of generalized systemic weakness and imperfect 

 development show themselves in connexion with those pro- 

 perties which differentiate man from the animals most nearly 

 allied, namely, in weakness and arrest of the higher functions 



1 See the chapters "On Growth and Overgrowth" and "On the Causation 

 of Cancerous and other New Growths " in Part III. of this volume. 



