INTRODUCTION. Xl 
The restriction of the dispersal of dominant species in certain directions 
may be satisfactorily explained by the presence of physical obstacles or by 
the competition to which they become subjected from other species of like 
habit and of an almost equal development. 
VariATION, whether in form, colouring, or habits, tends to be protective, 
and leads to general adaptability or to special adaptation, the former 
tending to advancement, and the latter towards degeneration, and such 
being the biological expression of the changes of the environment, are 
always worthy of attentive study, and the neglect that is usually accorded 
to so-called casual variations is quite unmerited, as these, if not atavic, 
probably indicate the direction in which the variation of the particular 
species or group is tending, or the aberrant individuals may, owing to the 
less pronounced character of the surroundings, be the outliers of the 
district or region where the particular form is the prevalent one; the 
latter case is well seen in the var. albolateralis of Arion ater which, 
though occasionally found in other parts of this country in diminished 
brilliancy and beauty, is in certain portions of North Wales very dis- 
tinctive and quite the commonest form of the species. In like manner 
the var. maculata of Limax arborum is quite rare and sporadic in 
England, but in Ireland is much more plentiful, but the causes of these 
aberrations have not yet been discovered. 
The change resultant from a different environment may be in the 
direction of warning colours or markings, as displayed by the vividly 
coloured forms of the var. rufu of Arion ater, which, by the deposition 
of the coloured excretory products within and upon the outer integument, 
have rendered themselves distasteful to former enemies, the vivid colour- 
ing acting as a warning indication of their probably nauseous character 
as food. 
On moist ground and upon the cloudy and mist-enveloped hills and 
mountains, a darker pigmentation of the body usually takes place, assimi- 
lating the animals more closely to the dark, damp aspect of the stones and 
rocks among which they dwell. ‘This effect is shown especially and strik- 
ingly by Limax arborum, and is also confirmed by Geomalacus maculosus 
and Arion ater var. aterrima, although the latter has also been reported 
as inhabiting low and swampy ground. 
In Agriolimax agrestis a tendency is shown during the autumn months 
for the animals to approximate in colour to the dead and fallen leaves so 
universally prevalent at that season of the year, the body of the animal 
becoming flecked over with brown, or even changing to a general brownish 
tint. 
