COERELATED VARIATIONS. 91 



are comparatively common, whilst other abnormalities 

 are rare. Typically one gonad (or reproductive organ) 

 occurs on each radial canal, but on an average about 

 one in seven of them fails to develop. In atypical 

 forms, however, as can be gathered from the table, the 

 proportion failing to develop is found to be larger and 

 larger, the further the departure of the individual from 

 the type. In such forms as depart from the normal 

 radial symmetry, even if they still possess five rays, the 

 partial sterility is very much increased. 



PER CENT. OF GONADS FAILING TO DEVELOP. 



NUMBER OP FREQUENCY OF IRREGULAR 



CANALS. OCCURRENCE. ALL INDIVIDUALS. INDIVIDUALS. 



2 1 



3 8 25.0 44.5 



4 56 21.8 31.0 



5 860 15.6 37.0 



6 64 18.6 31.0 



7 6 21.5 43.0 



8 1 



A very interesting case of variation described by Bate- 

 son * may perhaps be ascribed to the action of genetic 

 selection, though there is no direct evidence to warrant 

 the assumption. It concerns the Peppered moth, 

 Amphidasys betularia. A striking black variety of this 

 insect, A. doubledayaria, was first met with as a rarity 

 in 1840-50. Since then its numbers have gradually in- 

 creased, till in 1870 about equal numbers of the pure 

 type and of its variety occurred at Monmouth, whilst a 

 few years later the typical form had entirely disap- 

 peared. At Chester none but black forms have been 

 met with for many years. In the south of England, 

 however, the typical form is still alone present. Bate- 

 * Science Progress, vol. vi. p. 557, 1897. 



