Selective Pairing in Gammarids. 327 



ject touching the skin between the members of a pair, but instead 

 are pulled widely apart after such stimulation, with their terminal 

 combs retracted. 



By the action particularly of pilocarpine, it can be shown that 

 in species normally sluggish, responding mildly to external excita- 

 tion, the much more violent type of behavior characteristic of 

 species armed with urticant spines may be induced through the 

 effect of neurophil drugs. Therefore the effect of these substances 

 is brought about in relation to nervous pathways already existing. 

 And a suggestion is had as to the basis of behavior differences in 

 species structurally related. 



The failure of strychnine to produce its "typical" effects, in 

 these insects, coupled with the observed "reversal" under atro- 

 pine, points to possible chemical differentiation of the synaptic 

 homologues in insects, and argues for caution in the use of drugs 

 as a test for synapse-function in invertebrates. 



154 (1901) 

 Selective pairing in gammarids. 



By L. H. SNYDER and W. J. CROZIER. 



[From the Zoological Laboratory, Rutgers College, New Brunswick, 



N. J.] 



Studies on the sexual coupling of organisms have shown it 

 necessary to recognize that association of mates may be selective 

 rather than random. It is evident that such selective coupling 

 may have important evolutionary consequences. 1 The problem 

 of selective coupling on the basis of somatic characteristics, how- 

 ever, is an entirely different problem from that of selective union 

 of germ cells. This point must be clearly in mind; the distinction 

 has occasionally lapsed in discussions of the topic. 2 



It has been shown by Pearl and by Jennings that paramcecia 

 assort with respect to size ; and the nature and effects of this assor- 

 ting have been pointed out. More recently it has been shown 

 that the nudibranch Chromodoris zebra, which practises internal 



1 Wright, S., Genetics, 1921, vi, 144. 



8 Cf. Jones, D. F., Biol. Bull., 1920, xxxviii, 251. 



