THE PHENOMENA OF REVERSION 347 



this case by the fact that the unmated females retain the recep- 

 taculum seminis, which is unused and always empty. 



The two varieties must have originated at a time when sexual 

 reproduction occurred — at any rate periodically : were this not 

 so, primary constituents of A could not be present in the germ- 

 plasm of B, and vice versa. The co-existence of both kinds of 

 primary constituents in the same animal, can only be understood 

 if we suppose that sexual reproduction had occurred at no very 

 distant period. 



The explanation of the process of reversion naturally follows 

 from the fact that in species in wJiich parthenogenesis regularly 

 occurs., a reducing division nevertheless takes place, but only a 

 single one : ofie polar body is separated off from the egg, and 

 not two, as in sexual reproduction. This single halving of the 

 idants in the ovum must undoubtedly be preceded by a doubling, 

 just as occurs in sexual reproduction ; for a reduction of the 

 number of idants to one half would otherwise take place from 

 generation to generation, so that ultimately only a single one 

 would be left. If, however, a reducing division preceded by a 

 doubling of the number of idants takes place, reversion becomes 

 possible. 



Let us take a case of the simplest possible kind, and suppose 

 that there are four idants in the germ-plasm, three of which are 

 entirely composed of ids of the type A, and the fourth of ids of 

 the type B. The four idants a, a, a, b, first become doubled, so 

 that eight idants, a, a, a, a, a, a, b, b, result. Let us assume the 

 most favourable case for reversion towards the variety B to 

 occur, the reducing division resulting in the separation of these 

 idants into the groups a, a, a, a, and a, a, b, b, the latter of which 

 forms the nucleus of the egg-cell : the daughter individual arising 

 from this egg would then produce primary ova containing the 

 group of idants a, a, b, b. After the process of doubling in the 

 ripe ovum, this group would then have the composition a, a, a, a. 

 b, b, b, b ; and if the reducing division occurred in such a manner 

 as to result in the combination a, a, a, a, passing into the polar 

 body, and the combination b, b, b, b, remaining in the nucleus 

 of the ovum, an individual of the variety B would undoubtedly 

 arise from the egg, and reversion would ensue. 



These processes would in reality be much less simple, and 

 would take place much more slowly. For the number of idants 

 is doubtless in most cases much larger, and the addition of a 



