COCKERELL AND COLLINGE : CHECK-LIST OF SLUGS. 169 



person would set matters right. In this way Limax filans, Hoy, 

 L. squamtnatinus, 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. Limax maximus and L. flavus, for instance, are 

 subgenerically distinct ; and English authors having in mind such 

 distinctions as they present, have at various times refused to 

 recognise species or subspecies differing in less degree. Thus Avion 

 hortensis and A.fasciatus, Testacella haliotidea and T. scutulum, and 

 other valid species, have been long confounded. When we examine 

 the maximus group of Limax, the gagates group of Amalia, the icevis 

 group of Agriolimax, &c, the sharp distinctions between species 

 seem to be altogether lost. In Central Europe the subspecies or 

 species allied to Limax maximus 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 maximus, cinereo-?iiger, and geographicus, &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 Argynnis, of C. B. Adams on Jamaican Land 

 Shells, of Bebb and Buchanan White on willows, of Baker on roses, 

 &c, for similar instances. 



Whereas formerly slugs were described only from external 

 characters, the tendency now is to have little regard for any but 

 anatomical. Here there is undoubtedly danger of error, since it 

 is difficult to find out in many cases what is the stability of the 

 apparent anatomical distinctions. There are plenty of " anatomical 

 species " now on the lists which may hereafter be abolished 2 ; and 



1 The difference between these slugs is constant and well marked, and they were only 

 confounded so long as their structure was unknown. — W. E. C. 



- For every so-called " anatomical species" which may hereafter be abolished we may count 

 a dozen of the other species which most certainly will have to be placed as synonymns. — 

 W. E. C. 



