28o SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



breeding between all individuals is impossible, and hence the 

 obliteration of individual difTerences is also impossible. 



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

 first of all consider what happens in monogonic or unisexual 

 reproduction, which actually occurs in parthenogenetic organ- 

 isms. 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, re- 

 producing itself by the monogonic process and therefore with- 

 out cross-breeding, differ, not only in transient but also in 

 hereditary' characters. 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 diflerences 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 

 method, as in many species of Cynips and in certain lower 

 Crustacea. But all these differ from our hypothetical case in 

 one important respect ; it is always impossible for all the 

 descendants to reach maturity and reproduce themselves. The 

 vast majority of the descendants generally perish at an early 



