284 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



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 organisms which reproduce themselves sexually are 

 never without ancestors, and if these latter were also propa- 

 gated 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. 335). 



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 ques- 

 tion 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 com- 

 pelled to take them for granted, without any attempt to enquire 

 into their origin ? If we abandon this enquiry, we can never 

 achieve a complete 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 development 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) dif- 

 ferences 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 satis- 

 factory explanation. 



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

 origin of hereditary individual variability cannot indeed be 

 found in the higher organisms^the Metazoa and Metaphyta ; 

 but it is to be sought for in the lowest — the unicellular 



