IN THE THEORY OF NATURAL SELECTION. 277 



We are thus led to the conclusion that even in a few sexually 

 produced generations a large number of well-marked individuals 

 must arise : and this would even be true of generations springing 

 from our hypothetical species, assumed to be without ancestors, and 

 characterised by few individual differences. But of course organ- 

 isms which reproduce themselves sexually are never without 

 ancestors, and if these latter were also propagated by the sexual 

 method, it follows that each generation of every sexual species is 

 in the stage which we have previously assumed for the tenth 

 or some much later generation of the hypothetical species. In 

 other words, each individual contains a maximum of hereditary 

 tendencies and an infinite variety of possible individual characters 

 (see Appendix VI, p. 336). 



In this manner we can explain the origin of hereditary in- 

 dividual variability as it is known in man and the higher animals, 

 and as it is required for the theory which explains the transformation 

 of species by means of natural selection. 



Before proceeding further, I must attempt to answer a question 

 which obviously suggests itself. For the sake of argument, I 

 have assumed the existence of a first generation, of which the 

 individuals were already characterised by individual differences. 

 Can we find any explanation of these latter, or are we compelled 

 to take them for granted, without any attempt to enquire into their 

 origin ? If we abandon this enquiry, we can never achieve a com- 

 plete solution of the problems of heredity and variability. We 

 have, it is true, shown that hereditary differences, when they have 

 once appeared, would, through sexual reproduction, undergo de- 

 velopment into the diverse forms which actually exist ; but this 

 conclusion affords us no explanation of the source whence such 

 differences have been derived. If the external conditions acting 

 directly upon an organism can only produce transient (viz. non- 

 hereditary) differences in the latter, and if, on the other hand, the 

 external influences which act upon the germ-cell can only produce 

 a change in its molecular structure after operating over very long 

 periods, it seems that we have exhausted all the possible sources 

 of hereditary differences without reaching any satisfactory ex- 

 planation. 



I believe, however, that an explanation can be given. The origin 

 of hereditary individual variability cannot indeed be found in the 



