COCKERELL AND COLLINGE : CHECK-LIST OF SLUGS. 229 



this or that genus or the system of classification, all malacologists 

 will, 1 think, appreciate the amount of patient labour involved in such 

 an undertaking. 



Professor Cockerell has invited me to append to his list some 

 critical observations and notes of my own, but as the same has 

 passed through my hands during the vacation, during which period 

 I have had other work to complete, and often away from the 

 sources of literature desired, I have only been able to express an 

 opinion on a few general points. 



I am not aware that the slugs have ever been so catalogued 

 before, and therefore a very many points of difficulty must have 

 arisen to the compiler as to classification, priority, generic and 

 specific distinction, &c, in all of which cases a decision was a very 

 perplexing matter. No one appreciates more than myself Professor 

 CockerelPs work upon the slugs ; in fact, for the last five years I have 

 followed very closely his writings, and have been in constant 

 correspondence with him, and which, I hope, may continue for 

 very many years to come. We regard the slugs — in fact, the 

 Mollusca altogether — from two entirely different standpoints ; 

 nevertheless, we are prepared to agree to differ, and not to permit 

 objectional personalities to arise or stand in our way in elucidating 

 the history of so interesting and important a group. 



Hitherto the slugs have been studied purely from a systematical 

 standpoint, but with the publication of the works of Simroth, Semper, 

 Lessona, Pollonera, Godwin-Austen, Scharff, Hedley, and others, our 

 views are rapidly changing, and a new and more rational system is 

 supplanting the old. This new system — which I am pleased to 

 observe is spreading in other departments of Zoology 1 — demands 

 a knowledge of internal as well as external morphology, and as I have 

 previously stated, 2 rightly refuses to recognise inadequate descriptions or 

 descriptions of shells apart from the animal, or to acknowledge genera 

 or species founded upon purely external features ; in short, it demands 

 that they shall be classified and created " upon the aggregate 

 characters," and not upon single features. 11 



The slugs, as a group, are one which are subject to endless 

 variation in colour, markings, form, size, &c, &c. Mr. Gain has 

 shown that in a lifetime an individual species passes through a 

 number of distinct variations in colour, markings, and form, very 

 different, in some cases, from the adult animal. 4 The observations 



1 W. F. Kirby, "Nature," 1893 (roth Aug.), p. 339. 

 - Conchologist, 1892, vol. ii., p. 64 (footnc te). 

 ' ; Hedley, Trans. New Zeal. Inst., 1892, \xv., p. 15?. 

 4 Conchologist, 1892, vol. ii., p. 55. 



