IN THE THEORY OP NATURAL SELECTION. 289 



in this way that hereditary individual differences could arise and 

 persist. It was impossible for amphigony to disappear, for each 

 species in which it was preserved was necessarily superior to those 

 which had lost it, and must have replaced them in the course of 

 time ; ( for the former alone could adapt itself to the ever-changing 1 

 conditions of life, and the longer sexual reproduction endured, the 

 more firmly was it necessarily impressed upon the constitution of 

 the species, and the more difficult its disappearance became. 



Sexual reproduction has nevertheless been lost in some cases, 

 although only at first in certain generations. Thus in the Aphidae 

 and in many lower Crustacea, generations with parthenogenetic re- 

 production alternate with others which reproduce themselves by the 

 sexual method. But in most cases it is clear that this partial loss 

 of amphigony conferred considerable advantages upon the species by 

 giving increased capabilities for the maintenance of existence. By 

 means of partial parthenogenesis a much more rapid increase in the 

 number of individuals could be attained in a given time, and this 

 fact is of the highest importance for the peculiar circumstances 

 under which these species exist. A species of Crustacean which 

 inhabits rapidly drying pools, and developes from winter-eggs which 

 have remained dried up in the mud, has, as a rule, only a very 

 short time in which to secure the existence of succeeding genera- 

 tions. The few sexual eggs which have escaped the attacks of 

 numerous enemies develope immediately after the first shower of 

 rain ; the animals attain their full size in a few days and reproduce 

 themselves as virgin females. Their descendants are propagated in 

 the same manner, and thus in a short time almost incredible num- 

 bers of individuals are formed, until finally the sexual eggs are 

 again produced. If now the pool dries up again, the existence of 

 the colony is secured, for the number of animals which produce 

 sexual eggs is very large, and the eggs themselves are of course 

 far more numerous, so that in spite of the destructive agencies to 

 which they are subjected, there will be every chance of the survival 

 of a sufficient number to produce a new generation at a later 

 period. Here, therefore, sexual reproduction has not been abandoned 

 accidentally or from any internal cause, but as an adaptation to 

 certain definite necessities imposed upon the organism by its 

 surroundings. 



It is, however, well known that there are certain instances in 



u 



