214 MR. T. D. A. COCKERELL ON THE [A-pr. 7, 



every one that the bird is intermediate in its coloration between a 

 Mallard and a Gadwall. 



" The head, the wing, and tlie 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 Common Fowl. 



The following papers were read : — 



1, On the Geographical Distribution of Slugs. 

 By T. D. A. CocKERELL, F.Z.S. 



[Eeceived 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 ot 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 much 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 

 famil)-, as is still sometimes done. Of the six families of Slugs recog- 

 nized 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 Limaddce, which includes three families 



1 Art. MoUusca, reprinted from Encycl. Brit. 1891. 



