DISCONTINUOUS VARIATIONS 87 



The Moth Amphidasys. Some sixty years ago in the urban 

 conditions of Manchester the black variety doubledayaria of 

 the moth Amphidasys betularia found its chance, and soon 

 practically superseded the type in its place of origin, extended 

 over England, and appeared even in Belgium and Germany 

 (Bateson, 1905, p. 577). 



The Common Jelly Fish. A good case of abundant discon- 

 tinuity in variation is furnished by the common jelly-fish Aurelia 

 aurita, whose sports have been studied by eight or more ob- 

 servers, from Ehrenberg (1835) onwards. Its parts are normally 

 in multiples of 4 (4 equal areas in the radially symmetrical disc, 

 4 oral lips, 4 genital organs, 16 radial canals, 8 marginal sense- 

 organs or tentaculocysts) ; but numerical sports are very 

 common. These are sometimes irregular, e.g. when the radial 

 symmetry of the disc is lost ; but they are oftener quite sym- 

 metrical, e.g. when the animal has 2 genital organs, 2 oral 

 lobes, 8 radial canals, and 2 marginal sense-organs. 



In studying Aurelia aurita at Plymouth, Browne (1895) 

 found that out of 1515 young forms (ephyrae) 21^4 per cent, had 

 more or fewer than 8 marginal sense-organs, and that out of 383 

 adults 22 '8 per cent, were similarly affected. The figures seem 

 to show that the abnormal forms survive quite as well as the 

 normal forms, yet there is no evidence that the sports were more 

 numerous in 1895 than when Ehrenberg studied them sixty 

 years before. In other words, although a plentiful crop of 

 brusque variations is being continually supplied by this plastic 

 form, there is no hint of the origin of a new race. (Bateson, 

 1894, p. 428.) 



The Case of Pseudoclytia. Although the numerous discon- 

 tinuous variations of Aurelia aurita do not suggest that any new 

 race is at present arising, it is possible to find an analogous case 

 where it does seem that we have to do with a species newly 

 arisen, or still in process of being established. A. G. Mayer 

 found at the Tortugas, Florida, large numbers of a medusoid 



