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Show 214 MR. T. D. A. COCKERELL ON THE [Apr. 7, every one that the bird is intermediate in its coloration between a Mallard and a Gadwall. " The head, the wing, and the lower parts are specially noticeable, and the only conclusion one can come to is that it is a hybrid. Complete evidence on the subject of naturally bred hybrids of course it is almost impossible to obtain, but I do not see how this bird can be explained in any other way." Mr. E . T. Newton exhibited and made remarks on a small and abnormally shaped egg of the C o m m o n Fowl. The following papers were read:- 1. On the Geographical Distribution of Slugs. By T. D. A. COCKERELL, F.Z.S. [Received March 12, 1891.] The Slugs, or naked Land-Mollusca, are found in almost every inhabitable part of the globe, but the distribution of the several families and groups is much more restricted, presenting features of considerable interest. Salt water is fatal to Slugs, and it is evident that on land they are little fitted for extensive migrations, their tardy pace being proverbial. They frequent old logs and trunks of trees, and may very possibly be carried down rivers or even over short arms of the sea on floating timber; but, broadly speaking, their means of distribution may be said to be almost as small as those of any group of living organisms, not excepting the Mammalia and Amphibia. It follows, therefore, that their geographical distribution offers points of special value as bearing on questions relating to the former extent of land, and also, perhaps, to the climate of earlier times. For various reasons, which need not be discussed in the present paper, it is practically certain that at least the great majority of Slugs have descended from testaceous forms. This is especially clear in the Limacidce, where in Parmacella and other genera the young is m u c h more enclosed in a shell than the adult; while the life-histories of many of the slug-like Helicarionince are suggestive of a series of the still existing adult forms. It is also evident that the Slugs are of polygenetic origin, a fact which should prevent their being nearly all classed under a single family, as is still sometimes done. Of the six families of Slugs recognized in the present paper, five are more nearly allied to as many testaceous groups than to each other. The classifications of various authors are exceeding diverse, and especially does there seem to be the widest divergence of opinion among good authorities as to what constitutes a family. Thus Ray Lankester] gives a family Limacidov, which includes three families ] Art. Mollusca, reprinted from Encycl. Brit. 1891. |
