134 PROBLEMS OF GENETICS 



peregra has been known to occur for about fifty years. Visiting 

 it lately I found the left-handed shells to be about 3 per cent, of 

 the population. The species is the commonest British fresh- 

 water shell, but left-handed specimens are exceedingly rare. 

 Will anyone ask us to suppose that the persistence of a percentage 

 of this rarity in the same place is an indication of some specially 

 favouring circumstance in the waters of that pond? It is a 

 horse-pond to all appearances exactly like any other horse-pond ; 

 and I believe that in perfect confidence we may accept the 

 suggestion of common sense, which teaches us that there is 

 nothing particular in the circumstances which either calls such 

 varieties into existence or contributes in any direct way to their 

 survival. Had the phenomenon of local variation been studied 

 in detail before Darwin wrote, the attempt to make selection 

 responsible for fixity wherever found, could never have been 

 made. The proposition that not only the definiteness of local 

 forms but their variability also is sporadic, can be established 

 by countless illustrations taken from any group of either the 

 animal or the vegetable kingdoms. Only exceptionally can the 

 fixed differences be even suspected of contributing to adaptation, 

 and sporadic variability, which is a no less positive fact, must 

 manifestly lie outside the range of such suspicions. It is open 

 to any one to suggest speculatively that the persistence of 

 special varieties or of special variability in special places is an 

 indication that in those places the conditions of life are such 

 that the forms in question are tolerated though elsewhere 

 the same types are exterminated; but that consideration, even 

 if it could be proved to be well founded, is not one which lends 

 much force to the thesis that definiteness of type is a consequence 

 of Natural Selection. On the contrary, recourse to such reason- 

 ing implies the inevitable but very damaging admission that 

 the stringency of Selection is frequently so far relaxed that two 

 or more equally definite forms of the same species can persist 

 side by side. There is no doubt that this is the simple truth, 

 but when once that truth is perceived it is useless to invoke the 

 control of Selection as the factor to which definiteness of type 

 in general must be referred. 



