BELONGING TO DaPHNIA AND OI'HER ALLIED GENERA. 219 



of islets which it inhabits. With the exception of a green form, 

 exhibiting some structural modifications which I have dedicated 

 to our great herpetologist, under the name of Sphenodon Giin- 

 iheri^ it has been found impossible to distinguish these forms 

 except as local varieties, sufficiently well marked, however, to 

 admit of their being referred to their respective island habitats. 

 What are these, I would ask, but incipient species ? Allowing 

 sufficient time under the existing conditions of life, and reasoning 

 by analogy, each island or group of islets must in the end possess a 

 distinct species of Sphenodon exactly suited to its environment." * 



Mr. Gulick, in his study of the shells of the Sandwich Islands, 

 has found many well-marked local varieties, the result apparently 

 of a prolonged isolation. And in the case of very variable species 

 such as the common whelk, it would doubtless be possible for 

 an expert to determine with tolerable accuracy the localities in 

 which the more pronounced varieties in any collection had been 

 taken. But though varying to a very remarkable extent, it is 

 as yet impossible to point out definitely the distinctive local 

 characters of lacustrine Daphnice, such as D. galeata and D. 

 longispina. The subject, however, is well worthy of study and 

 would doubtless yield interesting results. 



In not a few cases sexual differences afford a valuable aid to 

 diagnosis, but in very many species the male has not yet been 

 observed, one of the most remarkable facts in the Natural History 

 of the Cladocera being the immense preponderance of females. 

 The experience of every collector teaches him that he constantly 

 finds ponds and ditches swarming with Daphnice, and yet does 

 not capture a single male smongst a whole netful of females. 

 The prevalent method of reproduction is, indeed, not sexual at 

 all but parthenogenetic, the female producing and detaching in 

 rapid succession broods of young which are the result of the 

 development, not of fertilized eggs, but of mere buds or *' pseud- 

 ova." The true fertilized eggs, when they occur, are con- 

 tained in peculiar saddle-shaped cases, which go by the name of 



* Illustrations of Darwinism ; or the Avifauna of New Zealand, considered in re- 

 lation to the fundamental Law of Descent with Modification, by Sir Walter L. Buller, 

 K.O.M.G., D.Sc, F.E.S.,&c. (Transactions of the New Zealand Institute, vol. XXVIL, 

 1894). 



