I.] INTRODUCTION. Xxi 



tral. The direction of the spire is attended by a corre- 

 sponding change in the position of those organs of the 

 animal which are usually placed on one particular side ; 

 and it may be compared to the case of a man having 

 his heart on the right and his lungs on the left side of 

 his body. The structure of a mollusk is, however, not 

 so complicated ; and the consequence of such a reversal 

 in the position of its organs is probably not very import- 

 ant to its economy. One curious case of this kind of 

 malformation is worth noticing. During the deposit of 

 that part of our upper tertiary strata which is called the 

 "Red Crag," nearly all the specimens of the almond 

 whelk (Fusus antiquus) appear to have been sinistral or 

 reversed, dextral specimens being comparatively very 

 scarce in this formation. The same species still exists 

 and is common in our seas ; but the proportion of dex- 

 tral to sinistral specimens is at present exactly the con- 

 trary to what it was in the Crag epoch the former being 

 now the rule and the latter the exception. A reversed spe- 

 cimen in a recent or fresh state is worth half a sovereign ; 

 while dextral specimens may be had at any street stall 

 (with the fish) at the rate of four for a penny. A few 

 bivalve species, which have one valve larger than the 

 other and are therefore called inequivalve, have also 

 their shells occasionally reversed, their right or left 

 valve being the largest, contrary to the usual rule in 

 these species. 



In the prosecution of any inquiry into the nature of 

 species or varieties, or as to whether there have been 

 any special or successive creations since this world was 

 called into existence by the fiat of its Great Creator, I do 

 not believe that it can have any irreligious or sceptical 

 tendency. Holy Scripture is not a work of natural his- 

 tory, and it is silent on the subject of this investigation. 



