270 W. J. CROZIER 



'purposeful adaptation.' This phase of the matter does not 

 seem to have received sufficient emphasis, although the scanty 

 observations available with reference to selective pairing contain 

 little evidence that such a process might be directly ''beneficial" 

 to a species. The case of Chromodoris zebra affords an instance 

 among metazoans which is exceptionally favorable for study. It 

 becomes, then, of special interest to discover, if possible, the 

 basis, or method, of the assortive mating practiced by this 

 nudibranch. 



2. Mating behavior of nudihranchs. Assortive mating with re- 

 gard to size would be expected to occur only where there is avail- 

 able some physical basis for the process of mutual selection 

 ('fitting') which is necessarily involved. According to Orton 

 (1914 a), the sea urchin Echinus miliaris, in its breeding season, 

 "has the habit of associating together in pairs;" the association 

 is often close, and in many instances the pairs found together 

 contain one individual of either sex. I have noticed instances 

 of this sort in Lytechinus variegatus, and several good examples 

 of the same nature have been observed by me with Tripneustes 

 esculentus. These cases have not yet been examined for the 

 occurrence of assortive pairing. In passing, it may be remarked 

 that if this type of "conjugation" is indeed at all general among 

 echinoids, it is apparently not sufficiently specific altogether to 

 prevent hybridization in nature (Shearer, de Morgan, and Fuchs, 

 '14). A possible basis might readily be conceived upon which 

 might be founded a correlation between the sizes of individuals 

 'pairing' in this way. Nevertheless it seems probable that in- 

 vertebrates which reproduce by external fertilization will not, 

 in general, yield evidence of assortive mating with respect to 

 size. Among nudibranchs, however, where a true copulation is 

 a prerequisite for the internal fertilization of the eggs,^ — as may 

 be demonstrated experimentally, and by the existence in some 

 forms, e.g., Doto, of recognizable anatomical arrangements mak- 

 ing for the prevention of self fertilization (Dreyer, '12, p. 342), — 

 there is a possibility that the mechanical conditions controlling 

 copulation may determine real homogamy as to size. This is 

 more especially true since, image-forming eyes being absent, 



