INTRODUCTION. Xlll 



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. albolateinlis of Arion ater which, 

 though occasionally found in other parts of this country in diminished 

 brilliancy and beauty, is in certain portions of N^orth Wales very dis- 

 tinctive and quite the commonest form of the species. In like manner 

 the var. maculata of Limax arhorum 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 tlie 

 direction of warning colours or markings, as displayed by the vividly 

 coloured forms of the var. riifa 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 Geomalaciis maculosus 

 and Arion ater var. aterrima, although the latter has also been reported 

 as inhabiting low and swampy ground. 



In AgrioUmax 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. 



