v.] IN- THE THEORY OF NATURAL SELECTION. 2)1?) 



Phyllopod crustacean, Apus, 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 some- 

 times 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 parthenogenesis has been first 

 introduced in alternation with sexual reproduction, the resting 

 eggs are produced by the latter generations, while the partho- 

 genetic generations produce eggs with thin shells, in which 

 the embr3^o 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 continue 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 rapidl}^ 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 appearance of the sexual genera- 

 tion and resting eggs. In all such cases the colony w^ould be 

 exterminated. 



This consideration might lead us to think that Crustacea, 

 such as the Daphuidae, which develope by means of heterogeny, 



