ORGANIC EVOLUTION — PHYSICAL 21 



ancestry, and therefore in cases in which evolution has 

 been rapid, as with race-horses, this reversion produces 

 wide divergences of structure from the parent, but in 

 cases in which the evohition has been slow, as with 

 lamp-shells, this reversion produces very slight diver- 

 gences of structure. For instance, to take an extreme 

 example, that amount of reversion which in the off- 

 spring of a man would result in a fish, would in the 

 offspring of a lamp-shell result in another lamp-shell, 

 only a little modified. For this reason it is that 

 characters long present in a race are, as is well known, 

 little capable of undergoing retrogression. Since race- 

 horses have been rapidly evolved under a process of 

 extremely stringent selection, reversion of the oJBFspring 

 to a comparatively recent ancestor causes a considerable 

 amount of divergence from the parent. Let us try to 

 imagine what would happen among race-horses if the 

 stringency of selection were relaxed, or rather if selec- 

 tion in this case were altogether abolished, so that the 

 inferior animals were allowed to propagate the species 

 as Avell as the superior. I think we may prophesy with 

 tolerable confidence. The selective action being with- 

 drawn, the tendency towards reversion, towards 

 atavism, would operate uncontrolled, and the race 

 would rapidly revert to its ancestor, the ordinary 

 horse. 



We are now in a position to understand why evolu- 

 tion caused by natural selection is so extremely slow. 

 In every species natural selection as a cause of evolution, 

 and atavism as a cause of retrogression, are constantly 

 at war. If the one force predominate we have evolu- 

 tion ; if the other, retrogression ; if the two forces 

 balance one another, the species in which this occurs 

 undergoes for the time neither evolution nor retrogres- 

 sion. Rapid evolution is accompanied by an increasing 

 tendency to retrogression, because in such a case any 



