253 



PRESIDENTIAL ADDEESS. 



VARIATION IN RECENT MOLLUSCA. 



By E. R. Sykes, B.A. 



Read lOth Fehruary, 1905. 



Variation in Zoology consists of the differences seen in animals, 

 and may be divided into two branches : the series under observation is 

 called continuous, where we pass from one form to another by im- 

 perceptible gi'adations, or the term discontinuous is used, where there 

 are marked gaps. 



Too much stress cannot be laid upon the importance of the subject, 

 and a true appreciation thereof is necessary both to the morphologist 

 and the systematist, while to the student of evolution it is the 

 foundation of his work. It is of special interest to the systematist, 

 since, without some knowledge of it, he is unable to properly utilize 

 the unit of his classification, a species ; and, indeed, he is frequently in 

 ignorance of what a species and its true limits may be. Eor instance, 

 Pisidium sinuatum, Brgt., is only an ordinary species with a sinuous 

 margin ; Calyptrca spirata is said to be a variation of C. Chinensis 

 which has taken its form owing to living in mud, where its sole 

 attachment is to Turritella; AmpuUaria Brohardi and Valuta Brazieri 

 are probably only monstrosities ; while those curious distortions found 

 in Planorhis have had nearly a dozen unnecessary specific names. 

 There is in the British Museum a specimen of Nautilus which, viewed 

 from one side is pompilius, while if it be examined from the other side 

 it appears to belong to stenomphalus. 



Again, the student of evolution desires to know how far the forms 

 seen to-day are constant, or in what manner they are affected by that 

 ever-present law of Nature, variation, progressive or retrogressive, as 

 it may be. It has been strongly urged that amongst the Achatinellidae, 

 for example, new species have been formed, and old ones have died 

 out, even in recent years. Generations of molluscs succeed one another 

 very quickly, and change of surroundings is liable to have a speedy 

 effect. These changes are taking place very rapidly at the present 

 day. In Saint Helena the bulk of the land fauna is extinct, owing 

 to the destruction of the trees, and similar conditions threaten many 

 other oceanic islands. In North America it has been noticed ' that 

 some of the woodland shells have been compelled to change their 

 habits and live in the open country. 



I would not suggest that every trifling variation be endowed with 

 a separate name, since that course must prove the road to chaos, but 



^ Billups: Nautilus, vol. xvi, p. 112. 



