304 Darwin, and after Darwin. 



"A special cauce may be assigned for the effects of use in' 

 causing hereditary atrophy of disused parts. It has already 

 been shown that all exceptionally developed organs tend to de- 

 teriorate : consequently, those that are not protected by selec- 

 tion will dwindle. The level of muscular efficiency in the wing 

 of a strongly flying bird [cui iously enough, the same case that 

 is chosen by I'rofessor Lloyd Morgan to illustrate his opposite 

 view], is like the level of water in the leaky vessel of a Danaid, 

 only secured to the race by constant effort, so to speak. Let 

 the effort be relaxed ever so little, and the level immediately 

 falls '." 



I take it, then, that the burden of proof lies with Professor 

 Lloyd Morgan to show why the withdrawal of selection is 

 not sufficient to account for degeneration any further than 

 the mediocrity-level in the former presence of selection. 

 Why does " the strong tendency ' to deterioration demand 

 an explanation," further than the fact that when all variations 

 below the average in every generation are allowed to survive, 

 they must gradually lower the average itself through a series 

 of generations ? To answer that any such tendency " would 

 have been bred out of the race " by the previous action of 

 selection, is to sujipose that the function of selection is at an 

 end when once it has built up a structure to the highest 

 point of working efficiency, — that the presence of selection 

 is no longer required to maintain the structure at that point. 

 Rut it is enough to ask in reply — Why, under the cessation 

 of selection, does complexity of structure degenerate so 

 much more rapidly than size of structure ? Why is it, for 

 instance, that "the eyes of Crustacea" in dark caves have 

 entirely disappeared, while their foot-stalks (when originally 

 present) still remain? Can it be maintained that " for 

 hundreds of generations " natural selection was more intent 



' A Theory of Heredity, Journal of Anthropological Institute, 1875. 

 Vol. V. p. 345. 



' No one has snpposed that the tendency need be "strong": it has 

 only to be persistent. 



