324 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



Thus, for example, the pure and exclusive parthenogenesis with 

 which the large Phyllopod crustacean, Apu&^ is reproduced at most 

 of its habitats, has not arisen from the loss of previously existent 

 sexual generations, but simply from the non-appearance of males, 

 accompanied by the simultaneous acquisition of the power, on the 

 part of the females, of producing eggs which do not require 

 fertilization. This is proved by the fact that males occur in certain 

 scattered colonies of this species, and sometimes they are even 

 present in considerable numbers. But even if we were not aware 

 of these facts, the same conclusions might nevertheless have been 

 drawn from the fact that Apus produces eggs of only one form 

 viz. resting eggs with hard shells. In every case in which par- 

 thenogenesis has been first introduced in alternation with sexual 

 reproduction, the resting eggs are produced by the latter genera- 

 tions, while the parthogenetic generations produce eggs with thin 

 shells, in which the embryo developes and hatches very rapidly. 

 In this way parthenogenesis leads to a rapid increase of the colony. 

 In Apus such increase in the number of individuals is gained in an 

 entirely different manner, viz. by the fact that all the animals 

 become females, which produce eggs at a very early age, and con- 

 tinue producing them in increasing fertility for the whole of their 

 life. In this manner an enormous number of eggs collects at the 

 bottom of the pool inhabited by the colony, so that after it has 

 dried up, in spite of loss from various destructive agencies, there, 

 will still remain a sufficiency of eggs to reproduce a numerous 

 colony, as soon as the pool has filled again. 



This form of parthenogenetic reproduction is especially well 

 suited to the needs of species inhabiting small pools which entirely 

 depend upon rain-fall, and which may disappear at any time. In 

 these cases the time during which the colony can live is often too 

 short to permit the production of several generations even from 

 rapidly developing summer- eggs. Under these circumstances the 

 pool would often suddenly dry up before the series of parthenogenetic 

 generations had been run through, and hence before the appear- 

 ance of the sexual generation and resting eggs. In all such cases 

 the colony would be exterminated. 



This consideration might lead us to think that Crustacea, such 

 as the Daphnidae, which develope by means of heterogeny, would 

 hardly be able to exist in small pools filled by the rain ; but here 



