86 THE FACTORS OP ORGANIC EVOLUTION. 



times increases its nutrition in proportion to its increased 

 action ; that while generating a modified consensus of 

 functions and of structures, the activities are at the same 

 time impressing this modified consensus on the sperm-cells 

 and germ-cells whence future individuals are to be pro 

 duced ; and that in ways mostly too small to be identified, 

 but occasionally in more conspicuous ways and in the 

 course of generations, the resulting modifications of one or 

 other kind show themselves. Further, it seems to me that 

 as there are certain extensive classes of phenomena which 

 are inexplicable if we assume the inheritance of fortuitous 

 variations to be the sole factor, but which become at once 

 explicable if we admit the inheritance of functionally-pro 

 duced changes, we are justified in concluding that this 

 inheritance of functionally-produced changes has been not 

 simply a co-operating factor in organic evolution, but has 

 been a co-operating factor without which organic evolu 

 tion, in its higher forms at any rate, could never have 

 taken place. 



Be this or be it not a warrantable conclusion, there is, 

 I think, good reason for a provisional acceptance of the 

 hypothesis that the effects of use and disuse are inheritable j 

 and for a methodic pursuit of inquiries with the view of either 

 establishing it or disproving it. It seems scarcely reasonable 

 to accept without clear demonstration, the belief that while 

 a trivial difference of structure arising spontaneously is 

 transmissible, a massive difference of structure, main 

 tained generation after generation by change of function, 

 leaves no trace in posterity. Considering that unquestionably 

 the modification of structure by function is a vera causa, 

 in so far as concerns the individual ; and considering 

 the number of facts which so competent an observer as 

 Mr. Darwin regarded as evidence that transmission of 

 such modifications takes place in particular cases ; the 

 hypothesis that such transmission takes place in con 

 formity with a general law, holding of all active structures, 



