rMS THE EVOLUTION THEORY 



and ]iow noeoHsnry, thorcl'ovo, tlie continual n'-combinatiou ol' tho 

 ids of the i;vnn-plasm by means of aniphiniixis must bo. \N o 

 understand why bi-sexnal repi'odviction was only abandoned in 

 one generation, and that tlie one in -wliich parthenogene.sis was of 

 considerable advantage. But such transformations must have come 

 nbout with extreme slowness, since they were the result ol' climatic 

 changes which only come abovit very gradually. Wo thus come 

 ftgain to the same conclusion to which we were led by our study 

 of vestigial organs in , Man, that numerous species ^vhich appear 

 to be at a standstill are continually working towards their own 

 improvement. But for this amphimixis is essential ; consequently 

 the descendants which have arisen through amphimixis, and whose 

 Ancestors have arisen in the same way, have an advantage over those 

 of parthenogenetic origin. On the whole, at least, this must be so; 

 in special cases it may be otherwise, namely, when the advantage 

 offered by parthenogenesis in respect to the maintenance of the 

 species preponderates over the advantage which amphimixis implies 

 as regards possibilities of transformation. 



As far as wc have seen from the case of the gall-wasps, the 

 Absence of amphimixis in every second generation implies no disad- 

 vantage in regard to the capability for transformation which the 

 species exhibits. As to whether any disadvantage would ensue if the 

 number ol' parthenogenetic generations in the life-cycle were greater 

 we can only guess, since no case is known which enabl(\M us to decide 

 this jioint, /irii or con, with any certainty. The heterogony of the 

 plant-lice, the Aphides, and their relatives might be cited as again.st 

 the probability, for in this case a long series of parthenogenetic 

 generations often alternates with a single bi-sexual one, but the 

 difference in structure is not so great in this case, although it does 

 exist, and moreover wc can quite well assume that the adaptation to 

 parthenogenesis was effected at the beginning of heterogony, when it 

 still consisted of a cycle of only two generations, and that further 

 virgin generations were interpolated subsequently. 



This assumption is supported by the fact that in some species of 

 our indigcaious Ostracods, in Cypris vidua and Camioiia cainlenn, in 

 contrast to the Daphnids, several bi-sexual generations alternate with 

 one parthenogenetic generation. But in this case again thei'e is no 

 difference whatever in the structure of the two generations, the 

 parthenogenetic gc^ieration being distinguished from the bi-sexual 

 generation simply by the absence of males. 



The alttntiation of generations in tho ])lant-lice is particularly 

 instructive, because it emphatically indicates how nuich Natui'e is 



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