ZOOLOGY: W. J. CROZIER 
519 
EVIDENCE OF ASSORTIVE MATING IN A NUDIBRANCH 
By W. J. Crozier 
BERMUDA BIOLOGICAL STATION FOR RESEARCH, AGAR'S ISLAND. BERMUDAi 
Communicated by E. L. Mark, June 11, 1917 
In man there is found, according to Pearson and others, a slight but 
appreciable degree of positive correlation between the members of 
mating pairs as regards their stature and certain other characters. 
For Paramecium a similar, but higher, correlation was proved by Pearl 
(1907) to exist between the lengths of members of conjugating pairs. 
Jennings (1911) substantiated Pearl's discovery that in Paramecium 
large individuals are usually found mated with large, small individuals 
with small, and made more certain the conclusion that this correlation 
(homogamy) is due to real assortive mating, as Pearl had previously 
maintained. 
This matter of assortive mating, which may have various important 
implications for evolution, appears not to have been studied in animals 
other than Paramecium and man. With reference to characters con- 
cerning the size of the organism, at least, it should, of course, be pos- 
sible for assortive mating to take place only when there is available 
some physical basis for the required process of selection. Hence, 
although echinoderms and some other marine animals appear to con- 
gregate at their times of breeding, and may even be conspicuously dis- 
posed in pairs (Orton) , it is not to be expected that invertebrates prac- 
ticing external fertilization would, in general, yield any evidence of 
assortive mating. Among gastropods, however, the case is different, 
and notably so with nudibranchs. In the latter animals, which are 
hermaphroditic, a true copulation of two individuals seems a prerequi- 
site for fertilization of the eggs. In some nudibranchs the male and 
female genital openings, two or three in number, situated on the right 
side of the body, are separated by a considerable distance, and the 
behavior of the animals in copulation shows that it is necessary for the 
^male' and 'female' openings of one individual to be brought simul- 
taneously into close relation with the appropriate openings of another 
(e.g., in Cenia, as described by Pelseneer, 1899). 
Other nudibranchs, such as chromodorids, have the reproductive 
openings concentrated upon a single small papilla; but, in some cases, 
at least, their behavior during the maneuvers preHminary to actual 
copulation strongly suggests that here also there is a rather well-defined, 
though not absolute, mechanical necessity for equality in the sizes of 
