328 



Scientific Proceedings (123). 



fecundation, shows a rather high degree of assortative mating ac- 

 cording to size. 1 Crozier pointed out some of the likely conse- 

 quences of large individuals mating with large, and small with 

 small, and he showed that an adaptive result is probably thus 

 attained in a purely mechanical, automatic manner. 





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Fig. 1. Relation between length of male Gammarus locusta and length of 

 female; measurements of 62 pairs. The dotted line shows curve of relation, if male 

 and female were of equal size in each pair. The line drawn through the plotted 

 points shows how the actual assortment depends in part on the intrinsically smaller 

 average size of the females. 



It is possible that this phenomenon, in varying degrees, is of 

 wide occurrence among animals. As further evidence there is 

 here presented a study of naturally occurring pairs of Gammarus 

 locusta (Linn.), a small salt-water crustacean. It is possible to 

 study size relations of the members of breeding pairs, because the 

 male carries the female about with him for a considerable time. 

 The formation of such pairs is by the usual sex-recognition meth- 

 ods of the Crustacea. It is agreed by those who have studied 

 sex- recognition in typical crustaceans that it is a purely mechani- 

 cal affair, 2 and this was observed to be true in Gammarus. The 

 male shows certain strong clasping reflexes, and itself resists being 

 clasped. The female, however, is usually passive when clasped. 



1 Crozier, W. J., Jour. Exper. Zodl., 1918, xxvii, 247; Amer. Nat., 1920, v, 182. 

 a Holmes, S. J., Biol. Bull:, 1903, v. 288; ibid., 1909, xvi, 313. Andrews, E. A., 

 Jour. Exper. Zobl., 1910, ix, 235, 



