266 THE EVOLUTION THEORY 



a variation movement originating in the germ-plasm may go on 

 increasing till it attains to selection-value, and then ' personal selec- 

 tion' steps in, and seeks to make it the common property of the 

 species. But it is obviously also conceivable that variational ten- 

 dencies arising in the germ-plasm may never attain to selection- value 

 at all, and then in most cases they will only continue to exist through 

 a longer or shorter series of generations as individual distinguishing 

 characters, Avithout being transmitted to a larger number of individuals 

 or becoming a constant character of the species. Their persistence 

 will depend essentially on the chance of mingling with other indi- 

 viduals, and on the halving of the germ -plasm which precedes sexual 

 reproduction. Sooner or later these individual peculiarities disappear 

 again, as may often be observed in the case of abnormalities or morbid 

 tendencies in man, in as far as these do not weaken vitality. In the 

 latter case they attain selection- value, though only negatively. 



But even quite inditterent germinal variations, which neither raise 

 nor lower the individual's joower of survival, may, under some circum- 

 stances, increase and lead to permanent variations of all the individuals 

 of a species, and this happens when they are conditioned by external 

 influences which atfect all the individuals of a species, or of the parti- 

 cular colony concerned, and it is this kind of organismal change which 

 we shall now study for a little in detail. 



The ordinary never-ceasing, always active germinal selection 

 depends, we must assume, upon intra-germinal fluctuations of nutri- 

 tion, or inequalities in the nutritive stream which circulates within 

 the germ-plasm. The variations wliich it produces may, therefore, be 

 diflerent in each individual, since these fluctuations are a matter of 

 chance and may aflect the determinants A in one individual and the 

 determinants B, C, or A'^ in another, or alternating groups of these. 

 Or it may be that the homologous determinants A may vary in a plus 

 direction in one individual, and in a minus direction in another, while 

 in a third they may remain unchanged, and although the same direction 

 of variation of a determinant X may occur in many individuals, it will 

 certainly not do so in all, and still less will it occur in all along with 

 the same combination of fluctuations in the rest of the determinants. 

 It is only if this occurs that the variation can become a specific 

 character. 



We might expect on a priori grounds that not only the chance 

 fluctuati(ms of nutrition within the germ-plasm would cause its elements 

 to vary in this or that direction, but that there would also be influences 

 of a more general kind, especially those of nutrition and climate, whicli 

 would in the first place aflect the body as a whole, but with it also the 



