On the Mechanical Conception of Nature. 657 



I now turn to the consideration of the second 

 factor of the theory of selection heredity. This 

 also, according to Von .Hartmann is not a me- 

 chanical principle. Darwin himself has now 

 become convinced how great is the probability 

 against the hereditary retention of modifications 

 which, whether feebly or strongly pronounced, 

 appear only in single individuals, i. e. of those 

 so-called " fortuitous " variations which are not the 

 expression of a directive developmental principle. 

 " But as among the numberless possible directions 

 of an indefinite variability, useful modifications can 

 only occur in single cases, Darwin has by this 

 supplementary admission himself retracted an 

 inadmissible assumption of his theory of selec- 

 tion," and so forth. A " regular, designed 

 tendency to variation, acting from within and 

 contemporaneously affecting a large number of 

 individuals," must therefore be assumed " in 

 order to insure the by itself improbable inheri- 

 tance." 



But even from the unbounded variability laid 

 down by the author, it by no means follows that 

 useful variations can only occur in single indi- 

 viduals. In the whole category of quantitative 

 variations the reverse is always the case. Is it 

 the lengthening of some part that is concerned ; 

 so would a large number of individuals always 

 possess the useful variation, since we are not 

 dealing with an absolute enlargement, but only 



