182 HEREDITY AND SELECTION IN SOCIOLOGY 



call species — that is to say, a number of individuals resembling 

 one another in all important features, differing from one another 

 only in unimportant variations, but marked off by a constant 

 difference in some important features from the individuals com- 

 posing other species ; and how it is that we do not find, instead 

 of these well-defined species, a number of loosely defined varia- 

 tions. For species, according to Niigeli, arc the result of the 

 action of a phyletic force moving always in a given direction, no 

 matter what may be the external conditions. 



But while there is an element of truth in Nageli's theory, it 

 cannot stand by itself. No phyletic force, scientifically conceived, 

 acting by means of purely intragerminal variations, is suflicient 

 to explain the evolution of the organic world as we know it. 

 But it is none the less true that the origin of every variation in 

 the organic world is found to be within the germ-plasm ; and 

 that every variation is initiated by a perturbation of the equili- 

 brium of the germ-plasm, a perturbation caused by oscillations 

 in the regulation of intragerminal nutrition. Thus, IJarwin'H 

 theory of natural selection as the sole cause of organic evolution 

 is supplemented by Nageli's conception of the internal origin 

 of variations. 



Support has recently been lent to the doctrine of Nageli by 

 the mutations theory of De Vries.^ In his study of various 

 cultivated plants— e.g-., (Enothera Lamarckiana — De Vrics found 

 that a number of strongly marked variations suddenly mani- 

 fested themselves in a number of individuals, and that these 

 variations bred true — that is to say, the variants, when ferti- 

 lised by their own pollen, brought forth the same variation in 

 identical form. All the Linnaaan species, acording to Do Vries, 

 are composed of a number of elementary species — such as the 

 sudden and spontaneous variation of (Enothera represents— 

 and these elementary species, in their turn, are the result, as is 

 proved by the case of (Enothera, of " mutation," of a sudden 

 ^ Hugo De Vrioa, Die Mutationatheorie. Leipzig, 1901. 



