6 HiCKLING, Variation of Planorbis umltifo7'niis, Bronn. 



curve that we are dealing with a single species. But the 

 three curves differ strikingly in the fact (most obvious in 

 Fig. 3) that the maximum or "mode" of the curve is 

 displaced in the direction of increasing height with each 

 increase in size. In other words, throughout life, as the 

 shell grows, the ratio height : diameter of base increases. 

 This fact would be much more striking if it had been 

 possible to construct a curve for shells less than 'oS in 

 diameter, for these would fall almost entirely into classes 



/■/- 



^. J. Curves as in Fig. 2, but smoothed by taking 

 the sums of overlapping pairs. 



A, B and C, with the maximum in B. Unfortunately I 

 had not a sufficient number of specimens so young, but 

 those at m)' disposal clearly indicate what the result would 

 be, while it is readily confirmed by an examination of the 

 apical portions of the adult shells. By the last-named 

 method of enquiry it may be shown that every individual 

 begins life as a discoid shell — a typical Planorbis. In a 

 general way, therefore, these changes in the form of the 

 variation-curve as the shells increase in size is merely a 

 graphic representation of the fact that the young shell i'; 

 a discoid Planorbis which acquires a turreted character 

 during growth, a change which must, without doubt, be 



