ZOOLOGY: W. J. CROZIER 
521 
ence sufficiently evident. Measurements of these pairs are plotted in 
figure 2. The correlation between the members of mating pairs is in 
fact better than in the previous case (fig. 1); reasons for this will be 
discussed in a subsequent paper. 
A study of the behavior of Chromodoris supports the view that there 
is exercised an active selection of mating partners. As a rule, two ani- 
mals greatly differing in size do not successfully copulate. This has 
Fig. 1 Fig. 2 
FIG. 1.— ILLUSTRATING THE CORRELATION IN SIZE BETWEEN THE MEMBERS OF 148 PAIRS 
OF CHROMODORIS ZEBRA COLLECTED IN THE FIELD 
The thin continuous line (1) gives the lengths (as ordinates) for the different classes of 
individua's in order of decreasing size. The heavy line (2) gives the average lengths of the 
mates of the individuals of these classes. The unit is 1 cm; ni' — is the mean for all, 
FIG. 2.— ILLUSTRATING THE CORRELATION IN SIZE BETWEEN THE MEMBERS OF 119 PAIRS 
OBTAINED IN LABORATORY EXPERIMENTS 
The thin continu"ous line (1) gives the lengths (as ordinates) for the different classes of 
individuals in order of decreasing size. The heavy line (2) gives the average lengths of 
the mates of these classes. The unit is 1 cm; m — m' is the mean for all. 
been verified by experiments in which the size of some individuals 
has been artificially reduced through starvation. The physical basis 
of assortive mating in Chromodoris is probably found in the relative 
attitudes assumed by the conjugants and in reactions to tactile (and 
chemical?) stimuli which determine these attitudes. 
Two suggestions may be made regarding the possible significance of 
