248 W. J. CROZIER 



usually quite evident to the eye when the pair is viewed through 

 a water glass. Two animals were considered to constitute a 

 mating pair when they were found together in the proper rela- 

 tive attitudes and were seen to be stationary in these attitudes 

 for some minutes. Copulating individuals remain in close con- 

 tact for a relatively long time, in some cases several hours. 



It is true that when collecting in a fathom or more of water 

 from a boat, mistakes may be made in judging mating couples, 

 although I believe that this error has not been great. Because 

 of this possibility, however, and a desire to observe more closely 

 the details of behavior in copulation, with reference to the pos- 

 sible mechanism of assertive mating (should this condition be 

 found to exist), experiments were performed under laboratory 

 conditions with a large number of individuals. 



The pairs collected in the field, and those obtained in laboratory 

 matings were measured in the living condition. Analysis of the 

 dimensions of the individuals found spontaneously copulating 

 amply confirms the impression derived from the inspection of 

 mating couples under natural conditions. A moderately high 

 degree of correlation between the sizes of the two individuals 

 forming a mating pair is evident in the measurements obtained. 

 This can only be understood in terms of the conclusion,- — which 

 is substantiated by the study of individual behavior in conju- 

 gation, — that assortive mating with respect to size (? age) is 

 practiced by Chromodoris zebra. 



The magnitude of the correlation index for the 'total length' 

 measurements is somewhat greater than the average known for 

 cultures of Paramecium containing a mixture of races. The 

 principal significance of assortive mating in Chromodoris is, 

 however, probably different from that assigned to homogamy in 

 Parameciimi. (Jennings). It seems that in C. zebra the imme- 

 diate result of assortive mating is to increase the number of 

 larvae beyond that which could be established by random 

 pairing. 



From the standpoint of species survival and dispersal, granted 

 the condition that animals of very different sizes are sexually 

 mature, this effect of homogamy is distinctly purposeful. It is 



