870 ENVIRONMENTAL STUDIES ON THE LIMPET. [NoV. 26, 



that surface ; or if the surface be quite smooth, the edge of the 

 shell becomes smooth and regular too. 



The difference between the ribs of the two types is to be 

 explained in the same way. All limpets below 15 mm., whether 

 rough or smooth, have about 12 principal ribs. These ribs are 

 very distinct in the rough shell, and project at the rim, giving a 

 very irregular outline to the shell (text-figs. 226 & 228). In the 

 smooth shell these ribs are inconspicuous and hardly project at all, 

 so that the outline of the shell is regular (text-figs. 225 & 227). The 

 specially large ribs of the rough shell arise primarily as projections 

 of the rim, mechanically caused by the irregularities of the rock- 

 surface. These projections of the rim must, owing to the very 

 nature of the shell's growth, be retained during growth, and 

 hence must be continued as i-ibs. In the smooth shell there is no 

 mechanical formation of projections of the rim, and hence there 

 are no specially prominent ribs. 



This case of the rough and the smooth limpet-shell is of interest 

 in that it shows that a small change in an environment may 

 produce through its continuous action a large cumulative result, 

 by a summation of successive little effects. " Continuity " of 

 environment may be apparent only, not real. 



References. 



(1) Bouchard-Chantereaux. — Catalogue des Mollusques marins 



.... du Boulonnais. Boulogne, 1835. 



(2) CooKE, A. H.— Camb. Nat. Hist. iii. (1895). 



(3) Davis, J. R. A.— Nature, xxxi. (1885) p. 200. 



(4) Idem, I. c. li. (1894) p. 512. 



(5) Davis & Fleure. — " Patella." Liverpool Marine Biological 



Committee, Memoir 10 (1903). 



(6) Fischer, H.— Journ. de Conchyl. xlvi. (1898) p. 314. 



(7) Fischer, P.— Manuel de Conchyliologie. Paris, 1887, 



p. 867. 



(8) Hawkshaw, J. C. — Journ. Linn. Soc, Zool. xiv. (1879) 



p. 406. 



(9) Jeffreys, J. G.— Brit. Conchologv, iii. (1865) p. 237. 



(10) Ltjkis, F. C— Mag. Nat. Hist. iv.'(1831) p. 346. 



(11) Morgan, C. Lloyd.— Nature, li. (1894) p. 127. 



(12) Norman, A. M.— Zoologist, I860. 



(13) Roberts, G.— Ann. Nat. Hist. xix. (1847) p. 70. 



(14) Robertson, D. — Trans. Nat. Hist. Soc. Glasgow, new ser. 



i. (1883-86) p. 9. 



(15) WiLLcox, M. A.— Amer. Nat. xxxix. (1905) pp. 325-33. 



