X1V INTRODUCTION. 
Limax 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 isa 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 the more extreme 
climatic changes to which the world has 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 Hisrory and progress of Limacology in these islands may be 
studied by enumerating in chronological sequence the species so far 
established as British, 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 the 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. 
‘he 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. 
he earlier period of activity is identified with the 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 the general investigation of our terrestrial mollusca. 
