INIENSIVK SEaKEGATION. 353 



maiutain that the very essence of the Darwinian theory is " that 

 specific differences must be advantageous," and therefore adapta- 

 tional ; while they do not claim the same for generic, family, and 

 ordinate distinctions, or indeed for varietal distinctions, if I rightly 

 understand *. I have never seen any attempt to explain this 

 supposed exception in the midst of the taxonomic series ; and it 

 seems to me that the break in the continuity of nature which 

 this interpretation of the Darwinian theory supposes, should 

 lead us to a very careful investigation of the facts before we accept 

 it as a true interpretation of nature. 



I shall coiitent myself with pointing out one distinction, oc- 

 casionally occurring between allied species, for which no use has 

 ever been, or is likely to be, found. 1 refer to the distinction 

 betv\een what are known as dextral and sinistral forms. This 

 distinction relates to the tortion of the animal and its shell upon 

 itself. It is moot easily recognized by placing the shell on its back 

 with the aperture upward, and observing whether the aperture 

 lies on the right side of the central columella of the shell or on 

 the left. In the first case it is described as dextral, in the second 

 as sinistral. In most families and genera of water-mollusks the 

 sinistral form occurs only as a sport (amongst Mammals the 

 heart is sometimes found on the right side), and even amongst 

 air-breathing mollusks the dextral form vastly predominates. 

 Amongst the AchatineUincB, Amastra and Leptachatina, which are 

 genera of terrestrial habits, are (with perhaps the exception of 

 one or two species) dextral in form; while the other genera, 

 which are plant-feeders and constantly hanging to branches or 

 leaves, present many species that are constantly sinistral, and 

 many others that are both dextral and sinistral. A¥hy should 

 Achatinella aclusta in Panoa and Makiki be constantly sinistral, 

 when its nearest allies found in the same valleys are both dextral 

 and sinistral? Why should Achatinella Dacca and A. ahhreviata 

 inPalolo and Waialae be constantly dextral when the other species 

 of Achatinella in the same valleys are for the most part sinistral ? 

 Is there any adaptation to the environment possessed by a dextral 

 form which would be lost if the form was reversed ? If not, 

 natural selection could not have anything to do with that part of 

 its character. Sulimella rosea is sinistral, while £. iulimoides is 

 dextral. If in this respect they should exchange forms, would 



* See letter from Mr. W. T. Thiseltou Dyer iu 'Nature,' vol. xxxix. p. 8. 



