NATURAL SELECTION AND ORGANIC VARIATION 49 



then natural selection again intervenes, and by removing those 

 individuals in which the variation is excessive, reduces it to 

 a level at which it is favourable to the species. It may even 

 happen that the life of such excessively specialised individuals 

 has to be artificially preserved by the breeder, as in the case of 

 the pigeons cited by Darwin, whose beak is too weak to open the 

 egg. Were the aid of the breeder removed, such unnatural 

 extremes would inevitably be eliminated. 



Thus the increase of an ascendant variation in a given part of 

 the organism is limited ; in the first place by the limited quantity 

 of nutritive substance in the germ-plasm ; and in the second place 

 by the bounds which natural selection sets, the overstepping 

 of which involves elimination. For regressive variation, on the 

 other hand, there is no limit set until the entire disappearance 

 of the weakened determinants has been effected ; and as soon 

 as a given determinant A has been eradicated from the germ- 

 plasm, the determinate A^ disappears from the soma. 



Regressive variation of the determinants is the cause of the 

 disappearance of superfluous parts of the organism. That this 

 is the case is a corollary of the conclusion that an organ, or a 

 part of an organ, can only be maintained in relative perfection 

 by means of natural selection. Natural selection operates by 

 eliminating those members of the species who do not possess 

 this organ, or who possess it in imperfect form. Once an organ 

 loses its value for the species, natural selection ceases to promote 

 its development or to ensure its persistence. Suppose a regres- 

 sive variation of this organ to be produced as the result of 

 perturbations of intragerminal nutrition, this inferior variation 

 will be able to reproduce itself under the same conditions as 

 the superior type, natural selection having ceased to have any 

 reason to favour the maintenance of the latter. On the contrary, 

 this organ having no further utility for the species, its mainte- 

 nance on the same level as that which it possessed when it was 

 useful is harmful to those individuals who do so maintain it. 



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