244 THE EVOLUTION THEORY 



and it ninst be so, since the egg-s, though secure against cohl and 

 desiccation, are very imperfectly protected against tl>e numerous 

 enemies which may do them injury. 



Of course the number of individiials which fonn a colony may 

 varj^ greatly in the ditterent species, and the same is true of the 

 number 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 favoni-able conditions, so that, 

 for instance, a species which li^■es in large lake-basins will produce 

 many purely pax'thenogenetic generations before the bi-sexual one, 

 which onlj' appears towards auttimn, while s[iecies which live in 

 quickly-drying pools have only a few parthenogenetic gener-ations, 

 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 

 genei-ations, and the result is that the uniformity of the germ-plasm, 

 which is the necessary consequence of pure pai'thenc^genesis, is inter- 

 rupted after a longer or shorter series of generations by the occin-rence 

 of amphimixis. That the number of parthenogenetic generations may 

 be so \'aried, 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 highest level, and that its influence 

 appears whether it occurs in the species regulai'ly, or frequently, or 

 only rarely. 



This kind of alternation of generations, that is, tlu' alternation 

 between unisexual (female) and bi-sexnal generations, has bt^en called 

 heterogony. In the Daphnids, certainly, a diiference in form betwiHMi 

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

 the same females which produce eggs requiring fei'tilization can also 

 produce parthenogenetic ova, although these are \ery ditlerent from 

 each other, as we haxe already seen. The difi'erence betwei^n 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 dilTerent generaticms diverge 

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

 that of the gall-wasps (Cynipidm). In many of these little Hymeno- 



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