414 DARWINISM chap. 



tlius been increased (or diminislied) by selection, there is in 

 the offspring a strong tendency to revert to a mean or average 

 size, which tends to check further increase. And this mean 

 appears to be, not the mean of the actual existing individuals 

 but a loAver mean, or that from which they had been recently 

 raised by selection.^ He calls this the law of "Regression 

 towards Mediocrity," and it has been proved by experiments 

 with vegetables and by observations on mankind. This regres- 

 sion, in every generation, takes place even when both parents 

 have been selected for their high development of the organ in 

 question ; but when there is no such selection, and crosses are 

 allowed among individuals of every grade of development, the 

 deterioration will be very rapid ; and after a time not only 

 will the average size of the part be greatly reduced, but the 

 instances of full development will become very rare. Thus 

 what Weismann terms " panmixia," or free intercrossing, will 

 co-operate with Gal ton's law of "regression towards mediocrity," 

 and the result will be that, whenever selection ceases to act on 

 any part or organ which has heretofore been kept up to a 

 maximum of size and efficiency, the organ in question will 

 rapidly decrease till it reaches a mean value considerably 

 below the mean of the progeny that has usually been produced 

 each year, and very gi-eatly below the mean of that portion 

 which has survived annually ; and this will take place by the 

 general law of heredity, and quite irrespective of any use or 

 disuse of the part in question. Now, no observations have been 

 adduced by Mr. Spencer or others, shoAving that the average 

 amount of change supposed to be due to disuse is greater than 

 that due to the law of regression towards mediocrity ; while 

 even if it were somewhat greater, we can see many possible 

 contributory causes to its production. In the case of civilised 

 man's diminished jaw, there may well be some correlation 

 between the jaw and the brain, seeing that increased mental 

 activity would lead to the withdrawal of blood and of nervous 

 energy from adjacent parts, and might thus lead to diminished 

 groAvth of those parts in the individual. And in the case of 

 pet-dogs, the selection of small or short-headed individuals 

 would imply the unconscious selection of those "vvith less 

 massive temporal muscles, and thus lead to the concomitant 

 ^ Journal of the Anthropological Institute, vol. xv. pp. 246-260. 



