Fig. 2. Regression plot; ordinates are the main lengths of the mates of males 

 in the corresponding length-classes. The index of correlation (length of male : length 

 of mate) is r — 0.804 ± 0.030. 



With the help of a student, Mr. Connemacher, pairs of Gam- 

 marus locusta were collected on the shore of Raritan Bay, on 

 Staten Island. The length of each of the members of each pair 

 was measured. The results show a high degree of selective coup- 

 ling on the basis of length. The figures bring this out clearly. 

 Large individuals tend to mate with large, and small with small. 

 This appears to be entirely automatic. 



The upper limit of length of the female with which any given 

 male will mate seems to be determined by his ability to clasp and 

 hold her. 1 The lower limit is much more elastic, but is neverthe- 

 less a limit, apparently also determined by mechanical features of 

 the clasping process. 



These animals can be bred in the laboratory, and it should 

 therefore be possible to discover the result or lack of result of the 

 selective coupling. 



1 Holmes, loc. cit. (1903). 



