Vol. XLV. August, i$2j. No. 2. 



BIOLOGICAL BULLETIN 



SELECTIVE COUPLING OF GAMMARIDS. 



W. J. CROZIER AND L. H. SNYDER, 

 ZOOLOGICAL LABORATORY, RUTGERS COLLEGE. 



The result of a method of breeding involving the formation of 

 pairs according to some system of assortment, rather than upon a 

 purely random basis, is readily seen to be of great importance, not 

 only for the foundation of racial diversities, but also for the con- 

 servation of genetic stability (Romanes, 1906; Pearl, 1907; 

 Wright, 1921). Actual instances, however, of assortative mating, 

 occurring under natural circumstances, have been little studied. 

 And the matter is of interest beyond its strictly genetic bearings ; 

 for it is probable that through sexual coupling selective with re- 

 spect to somatic features, there may in different species be achieved 

 various automatic, adaptive, consequences non-genetic in character, 

 but nevertheless significant racially (Crozier, 1918). In this re- 

 spect selective pairing of protozoans and of metazoans may differ 

 greatly as to their implications. The selective combination of 

 gametes (cf. Jones, 1920) is a question quite distinct from that of 

 assortment of individuals, and the two should not be confused. 



Among metazoans relatively few cases of normal selective mating 

 have been recognized, although Jennings (1920, p. 193) remarks 

 that the propensity for like to mate with like is probably in some 

 degree quite a general phenomenon. In Paramecium and allied 

 ciliates, which have been most extensively investigated, there is a 

 well-defined tendency toward conjugation between individuals re- 

 sembling one another in size and in fission-rate, and to this extent 

 at least structurally and physiologically akin (Pearl, 1907"; Jen- 

 ning, 1911, 1920; Watters, 1912). This is in large part due to 

 the fact that the mutual fitting of two individuals, requisite for 

 conjugation, is mechanically possible only when these individuals 

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