34 ESSAYS SCIENTIFIC AND PHILOSOPHICAL. 



triumphantly claimed by the Duke of Argyll as 

 " a great confession " of the failure of Darwinism. 



But Mr. Wallace points out (i.) that the in- 

 herited effect of use and disuse of parts is admitted 

 by Darwin in the " Origin of Species ;" and (ii.) that, 

 in the present state of knowledge, it is more than 

 doubtful whether both Darwin and Spencer were 

 not wrong in recognizing it at all. The instances 

 adduced can all be explained on the counter- 

 assumption of there being no inheriting of acquired 

 qualities, if we take into account the effects of the 

 withdrawal of the action of natural selection. 

 Where the struggle is going on every useful organ 

 is kept up to its highest limit of size and efficiency ; 

 but when the plant or animal is artificially protected 

 from the struggle for existence there is a natural 

 " regression to mediocrity," as Mr. Galton has 

 called it, which would explain, for instance, the 

 reduced size of the wings of many birds in oceanic 

 islands, as well as the diminished size of the 

 muscles used in closing the jaws in the case of pet 

 dogs fed for generations on soft food. According 

 to Herbert Spencer, this is due to the effect of disuse 

 independently of natural selection ; according to 

 Wallace and Darwin, and a fortiori Weismann, it 

 is due to the fact that an organ, abnormally in- 

 creased under certain circumstances by natural 

 selection, tends to revert to mediocrity when those 

 circumstances are changed. 



