IN THE THEORY OF NATURAL SELECTION. 273 



of individuals, because every individual possesses them, although 

 of a different kind and degree. The extinction of such dif- 

 ferences could only take place if a few individuals constituted a 

 whole species ; but the number of individuals which together 

 represent a species is not only very large but generally incalculable. 

 Cross-breeding between all individuals is impossible, and hence the 

 obliteration of individual differences is also impossible. 



In order to explain the effects of sexual reproduction, we will 

 first of all consider what happens in monogonic or unisexual re- 

 production, which actually occurs in parthenogenetic organisms. 

 Let us imagine an individual producing germ-cells, each of which 

 may by itself develope into a new individual. If we then suppose 

 a species to be made up of individuals which are absolutely identical, 

 it follows that their descendants must also remain identical through 

 any number of generations, if we neglect the transient non- 

 transmissible peculiarities caused by differences of food and other 

 external conditions. 



Although the individuals of such a species might be actually 

 different, they would be potentially identical : in the mature state 

 they might differ, but they must have been identical in origin. 

 The germs of all of them must contain exactly the same hereditary 

 tendencies, and if it were possible for their development to take 

 place under exactly the same conditions, identical individuals would 

 be produced. 



Let us now assume that the individuals of such a species, repro- 

 ducing itself by the monogonic process and therefore without cross- 

 breeding, differ, not only in transient but also in hereditary cha- 

 racters. If this were the case, each individual would produce 

 descendants possessing the same hereditary differences which were 

 characteristic of itself ; and thus from each individual a series of 

 generations would emanate, the single individuals of which 

 would be potentially identical with each other and with their 

 first ancestor. Hence the same individual differences would be 

 repeated again and again, in each succeeding generation, and 

 even if all the descendants lived to reproduce themselves, there 

 would be at last just as many groups of potentially identical 

 individuals as there were single individuals at the beginning. 



Similar cases actually occur in many species in which sexual 

 reproduction has been entirely replaced by the parthenogenetic 



