414 
DARWINISM 
CHAP. 
thus been increased (or diminished) 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 lower mean, or that from which they had been recently 
raised by selection. 1 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 Galton’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 greatly 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, showing 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 
growth 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 with less 
massive temporal muscles, and thus lead to the concomitant 
1 Journal of the Anthropological Institute, vol. xv. pp. 246-260. 
