Cockerell, T: Check-List of the Slugs (Classic Reprint)

Excerpt from A Check-List of the Slugs

L. Sgzzammatz'zzus, Morelet, &c., have for years been completely misunderstood Another difficulty relates to the limit of species. In England we find comparatively few species, and these for the most part very distinct from one another, so that we are hardly prepared for the numerous closely allied forms of Southern Europe and elsewhere. [jr/tax maxi/11115 and L. ¿avus, for instance, are subgenerically distinct; and English authors having in mind suich distinctions as they present, have at various times refused to recognise species or subspecies differing in less degree. Thus Arz'on lzorlezzsls and A. Fasoz'atas, T oslaoella lzalz'o/z'zlm and T. Sozzz'zzlum,' and other valid species,'have been long confounded. When we examine the maxi/nus group Of Lz'ozax, the gagates group Of Amalia, the law's group Of Agriolimax, &c., the sharp distinctions between species seem to be altogether lost. In Central Europe the subspecies or species allied to Lz'max maxi/nus are numerous and polymorphic, so that, despairing of finding good lines Of separation, authors have sometimes proposed to unite them under a single name. Yet to thus confound maxz'mz/s, oz'nereo-mgor, and geograp/zz'ms, &c., tends rather to obscure facts of great interest, and in the present list all such forms are given the rank of subspecies, which seems best to express their true standing. This gradation of forms, though so annoying to the pure systematist, is to the evolutionist full of interest, and the more carefully the units (whether species, subspecies, or varieties) are studied, the greater is the light thrown on the making Of species. The slugs are by no means exceptional in furnishing such evidence of evolution, and it is very instructive to notice the way in which the difficulties of classification have been met in different groups Of organisms. The student may be referred to the writings Of Allen and Merriam on North American Rodents, of W. H. Edwards on Argyzzm's, of C. B. Adams on Jamaican Land Shells, Of Bebb and Buchanan White on willows, of Baker on roses, &c., for similar instances.

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