140 REVERSION AND ALLIED PHENOMENA 



For all these cases, except that of the horse-stripings, as 

 also for similar cases given by Darwin, Mendelian interpreta- 

 tions are now forthcoming, and the hypothesis of there-assertion 

 of long latent ancestral characters is unnecessary. 



When the swimming-bell or medusoid Epenthesis folleata 

 appears with pentamerous symmetry instead of the usual 

 arrangement of its organs in fours or multiples of four, no one 

 would dream of calling this discontinuous variation an instance 

 of reversion, for we only know of one medusoid (Pseudoclytia 

 pentata) where five is normally the ruling number (Mayer, 1901). 

 But when the last-named medusoid occurs with four oral lips, 

 as it occasionally does, it may be said that this variation is 

 reversionary, since there is good reason to believe that Pseudo- 

 clytia pentata is a pentamerous derivative of the Epenthesis 

 stock. Even in this case the interpretation of the four lips 

 as reversionary may not be correct, since, as a matter of 

 fact, the number of lips in Pseudoclytia varies from one to 

 seven. 



Reversion in Parthenogenesis. — Weismann (1893, p. 344) 

 reports a very interesting case which he observed in varieties 

 of a small Ostracod crustacean (Cypris reptans) which multiplies 

 parthenogenetically. In the course of observations extending 

 over eight years he found that, amidst the expected uniformity 

 of resemblance between parent and offspring, exceptions occa- 

 sionally occurred. These were of such a nature that he could 

 only interpret them " as exhibiting reversions to an ancestral 

 form many generations removed." 



Other Instances of Reversion 



White-flowering Currant. — The white-flowering variety of the 

 red-flowering currant (Ribcs sanguineum) is said to have originated 

 many years ago from seed in Scotland. " Occasionally this white- 

 flowered currant reverts back to the original red type, and the 

 reversion takes place in the bud. . . . Once reverted, the branches 

 remain for ever atavistic. It is a very curious sight, these smal} 



