218 



HABIT AND INTELLIGENCE. 



[chap. XVII. 



Variation 

 held in 

 check by- 

 reversion. 



Tendency 

 to revert 

 may die 

 out with 

 lapse of 

 time, 



and limit 

 to varia- 

 tion may 

 recede. 



Eeason of 

 the possi- 

 bility of 

 this. 



stock, have ceased to vary in the direction of smaller size. The 

 reviewer infers from this that the dog has a sjiecilic limit as to 

 size, beyond which it is incapable of varying. 



I reply to this, that every form has a tendency to revert to 

 ancestral forms ; and the limit of smallness in the dog, or the 

 limit of any particular variation in any species, is at the point 

 where the tendency to farther variation is balanced by the 

 tendency to reversion. But, though the dog has at present 

 attained a limit at which the tendency to variation in the 

 direction of smallness is held in check, there is no proof that 

 such a limit is so grounded in the laws of life as to be abso- 

 lutely impassable. All tendencies come under the laws of habit, 

 and consequently may wear out and disappear with mere lapse 

 of time. If, consequently, the smallest breed of dogs now in 

 existence were kept separate and not permitted to mix with any 

 other for a sufficiently long time — say for a thousand years — 

 during which time all individuals that showed any tendency to 

 revert to the larger size of the original stock should be destroyed, 

 it is probable that the tendency to revert to the larger size 

 would disappear : the race might then begin again to vary in 

 the direction of smaller size ] smaller individual dogs might be 

 produced than any now in existence ; and, if selection were 

 applied to them, they would become the parents of a race of 

 dogs which would continually grow smaller, until the ten- 

 dency to variation was once more checked by the tendency 

 to reversion. 



The reviewer shows that he is aware of this argument, but 

 thinks that the assumption on which it rests — namely, that 

 mere lapse of time will obliterate the tendency to reversion — is 

 without any proof. 



Of course direct proof is out of the question. Geology gives 

 no evidence on such subjects; history is generally silent; tradi- 

 tion has forgotten them ; and experiment is impossible in cases 

 where an experiment might be only beginning to yield any 

 result at the end of a thousand years. But I think it is as 

 certain as such a thing can be, that all tendencies come under 

 the laws of habit, and that all unused habits and uumanifested 

 tendencies become weaker by mere lapse of time; though I 

 admit we can scarcely ever be certain that the tendency to 

 reversion has altogether died out. 



