v.] IN THE THEORY OF NATURAL SELECTION. 297 



has, as a rule, only a very short time in which to secure the 

 existence of succeeding generations. 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 pro- 

 duce 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 reproduc- 

 tion 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 which sexual reproduction has been altogether lost, and in 

 which parthenogenesis is the only form of propagation. In 

 the animal kingdom, such a condition chiefly occurs in species 

 of which the closely-allied forms exhibit the above-mentioned 

 alternation between parthenogenesis and amphigony, viz. in 

 many Cynipidae and Aphidae, and also in certain freshwater 

 and marine Crustacea. We may imagine that these partheno- 

 genetic species have arisen from forms with alternating methods 

 of reproduction, by the disappearance of the sexual phase. 



In any particular case, it may be difficult to point out the 

 motive by which this change has been determined ; but it 

 is most probable that the same conditions which originally 

 caused the intercalation of a parthenogenetic stage have been 

 efficient in causing the gradual disappearance of the sexual 

 stage. If a species of Crustacean, with the above-described 

 alternating method of reproduction (heterogeny), were killed 

 off by its enemies on a larger scale than before, it is obvious 

 that the threatened extinction of the species could be checked 

 by the attainment of a correspondingly greater degree of fer- 

 tility. Such increased fertilit}^ might well be produced b}^ pure 

 parthenogenesis (see Appendix V, p. 332), by means of which 



