235 



COLOUR VARIATION IN SOME BRITISH SLUGS. 



(Presidential Address delivered at the Annual Meeting, Oct. 17, 1908). 



By WALTER E. COLLINGE, M.Sc, F.L.S., F.E.S. 



In thinking over what subject I should choose for my Presidential 

 Address, it occurred to me that probably the subject I had most 

 recently been working at would be the one most likely to interest you. 



It is not my intention to enter upon so complicated a physiological 

 study as colour variation, even in so limited a group of animals as 

 our British slugs, but rather to describe a series of experiments which 

 have now extended over some years in connection with two or three 

 of our commonest British slugs. 



I have at different times pointed out that there is a tendency for 

 certain, if not all, species to vary in well-marked directions so far as 

 colour is concerned, and of these colour varieties various minor 

 colour variations are constantly taking place, due in part to food, 

 habitat, and other causes, but that such minor variations are not 

 constant, i.e., not perpetuated in the offspring. 



This opinion was largely founded upon observations during the 

 last twenty-five years, in examining large numbers of the colour 

 varieties of the different British species. So far as I am aware, there 

 is no actual experimental evidence proving or disproving this view, 

 and it therefore seemed highly desirable that a series of carefully- 

 planned experiments should be made with two or three common 

 species, and for this purpose I chose Arion evipiricorutn Fer. and 

 A. subfuscus Drap. 



In work of this character, as might only naturally be supposed, 

 many of the experiments were failures, owing to death, disease, and 

 other causes; a number, however, were most successful, and it is these 

 I wish to place on record. 



I. Arion empiricorum Fer. — As is well known to all students of 

 the slugs, this large black species has a white, a brown, and a red 

 variety, and also a series of varieties in which the dorsum is different 

 in colour from the lateral portions of the body. Such varieties are : 

 alba Linn., castanea Dum. and Mort., rufa Linn., and albolateralis 

 Roebuck. 



In the first experiment, eggs deposited by the var. casiaftea paired 

 with the typical form were taken. Of thirty-nine eggs, twenty-four 

 specimens were liatched and reared to maturity; and of these, twelve 

 were typical of the species excepting for slight variations on the foot- 



