XIV INTRODUCTION. 



Liinn.r tenellus by its yellow or pallid colouring, without noticeable 

 lateral banding, assimilates very closely to the aspect of the fungi upon 

 which it lives, and is not readily perceptible in such situations. 



External or tegumentary variation is quickly responsive to the changes 

 of environment, and the various colour mutations undergone in process of 

 growth by various of the naked species are colourings which were not 

 improbably beneficial and protective in former times, and it is very signifi- 

 cant as tending to shed a light upon their true evolutionary centre that 

 certain atavic varieties of many species characterized by the retention of 

 juvenile colours or markings are more prevalent at the confines of their 

 distribution than near the probable point of origin of the species. 



Structural modification is a much slower and more deliberate process, 

 and though undoubtedly proceeding everywhere, is much more rapidly 

 accomplished in the European region, which is, and has been for ages 

 past, the centre of the greatest evolutionary activity and the focus from 

 which improved forms of life have emanated and spread over the whole 

 surface of the globe, only interrupted by the rigours of tlie more extreme 

 climatic changes to which the world lias been from time to time exposed 

 during the progress of geological time, or by the varying dispositions of 

 land and water, which, however, would in many cases tend to accelerate 

 and facilitate dispersal. 



The History and progress of Limacology in these islands may be 

 studied by enumerating in chronological sequence the species so far 

 established as Briti.sh, and it seems on the whole better, as a simple act 

 of justice to the acumen and perspicuity of the original investigators, to 

 base the account upon the order in which the various species were 

 definitely introduced for tiie first time into the British fauna, rather than 

 to give the honour to the modern limacologists, who afterwards con- 

 firmed the truth of their predecessors' discernment by the demonstration 

 of structural and other differences. 



The existence of certain species in this country was, however, in several 

 instances foreshadowed by some of the older and more careful writers long 

 before their instatement in our lists. 



The history of Limacology in the British Isles may for the present pur- 

 pose be resolved into a modern and an earlier period of activity, separable 

 by a certain interval of neglect. 



The earlier period of activity is identified with tlie great names of 

 Lister, Alder, Gray, Johnston, and Clarke, although the slugs were not 

 specialized, except by the last-named author, being merely studied in the 

 course of tlie general investigation of our terrestrial mollusca. 



