778 THE POPULAR SCIENCE MONTHLY. 



scarcely to be resisted, that tlie inheritance of functionally-produced 

 mod iH cat ions takes place universally. Looking at physiological phe- 

 nomena as conforming to physical principles, it is difficult to conceive 

 that a changed play of organic forces which in many cases of differ- 

 ent kinds produces an inherited change of structure, does not do this 

 in all cases. The implication, very strong I think, is that the action 

 of every organ produces on it a reaction which, usually not altering 

 its rate of nutrition, sometimes leaves it Avilh diminished nutrition 

 consequent on diminished action, and at other times increases its nu- 

 trition 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 produced ; and 

 that in ways mostly too small to be identified, but occasionally in more 

 conspicuous ways and in the course of generations, the resulting modi- 

 fications 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 fimctionally-produced 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 evolution, 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 ; and for a methodic pursuit 

 of inquiries with the view of cither establishing it or disproving it. 

 It seems scarcely reasonable to accept without clear demonstration, 

 the belief that while a trivial difference of structure arising sponta- 

 neously is transmissible, a massive difference of structure, maintained 

 generation after generation by change of function, leaves no trace in 

 posterity. Considering that unquestionably the modification of struct- 

 ure 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 modifi- 

 cations takes place in particular cases ; the hypothesis that such trans- 

 mission takes place in conformity with a general law, holding of all 

 active structures, should, I think, be regarded as at least a good work- 

 ing hypothesis. 



But now supposing the broad conclusion above drawn to be 

 granted — supposing all to agree that from the beginning, along with 

 inheritance of useful variations fortuitously arising, there has been 

 inheritance of effects produced by UvSe and disuse ; do there remain no 

 classes of organic phenomena unaccounted for? To this question I 



