SELECTIVE COUPLING OF GAM MARIOS. 



99 



male. The length of each member of every pair secured was 

 measured under low magnification, with the aid of an ocular mi- 

 crometer; the total length, from anterior margin of cephalothorax 

 to posterior margin of last abdominal segment, was measured along 

 the curved dorsal outline. 



10 II 12 13 14 15 16 17 mm. 



FIG. 2. Relation between length of male and of associated female in 71 

 pairs of Dikerogammarus fasciatns; lengths in nuns., for each pair; ordinates, 

 lengths of females ; abscissas, lengths of males. 



The relation of the length of males to that of the associated 

 females, in Gammanis locusta, is shown in Fig. i (Snyder and 

 Crozier, 1922). Fig. 2 contains the corresponding observations 

 for Dikerogammarus fasciatns. There is in each instance a high 

 degree of selective coupling on the basis of length. For G. locusta 

 the correlation index for size among members of mating pairs is 



