244 THE EVOLUTION THEORY 



and it must be so, since the eggs, though secure against cold and 

 desiccation, are very imperfectly protected against the numerous 

 enemies which may do them injury. 



Of course the number of individuals which form a colony may 

 vary greatly in the different species, and the same is true of the 

 numljer of parthenogenetic generations which precede the bi-sexual 

 generation. I have already shown in detail that this depends 

 precisely on the average duration of the favourable conditions, so that, 

 for instance, a species which lives in large lake-basins will produce 

 many purely parthenogenetic generations before the bi-sexual one, 

 which only appears towards autumn, while species which live in 

 quickly-drying pools have only a few parthenogenetic generations, 

 and the true puddle-dwellers give rise to males and sexual females 

 along with the parthenogenetic females as early as the second 

 generation. 



We thus find in the Daphnids an alternation, regulated and made 

 normal by natural selection, of purely parthenogenetic with bi-sexual 

 generations, and the result is that the uniformity of the germ-plasm, 

 which is the necessary consequence of pure parthenogenesis, is inter- 

 rupted after a longer or shorter series of generations by the occurrence 

 of amphimixis. That the number of parthenogenetic generations may 

 be so varied, though with a definite norm for each species, indicates 

 again that amphimixis is not an absolute condition of the mainten- 

 ance of life, not an indispensable rejuvenation, designed to counteract 

 the exhaustion of vital force — whether this be meant in a transcen- 

 dental sense or otherwise — but that it is an important advantage 

 calculated to keep the species at its higliest level, and that its influence 

 appears whether it occurs iri the species regularly, or frequently, or 

 only rarely. 



This kind of alternation of generations, that is, the alternation 

 between unisexual (female) and bi-sexual generations, has been called 

 heterogony. In the Daphnids, certainly, a difference in form between 

 the parthenogenetic and the bi-sexual generation does not exist, for 

 the same females which produce eggs requiring fertilization can also 

 produce parthenogenetic ova, although these are very different from 

 each other, as we have already seen. The difference between genera- 

 tions, therefore, does not lie in their structure, but in their tendency 

 to parthenogenetic or to amphigonous reproduction, and in the absence 

 or presence of male individuals. There are, however, other cases of 

 alternation of generations in which the different generations diverge 

 from each other in structure. One of the most remarkable of these is 

 that of the gall-wasps (Cjaiipidre). In manj- of these little Hymeno- 



