400 T. Bland on the Geographical Distribution of Mollusca. 



may very justly be added, — the confined views entertained by 

 authors as to the origin of species. 



With respect to the first, Dr. Gould remarks truly : — 



'' A voyage is made to the Sandwich Islands, and all the shells 

 brought home by the vessel are said to be shells from the Sand- 

 wich Islands, though they may have been obtained at California, 

 the Society Islands, New Zealand, and, perhaps, half a dozen 

 other places quite as remote from each other. A sea captain pur- 

 chases a collection at Calcutta or Valparaiso, for his friends at 

 home ; and all the shells are marked as denizens of the port where 

 they were purchased, though they might not have lived within 

 thousands of miles. Purchased shells cannot be relied on for 

 localities ; for this end a shell must have been found containing 

 the animal, or else dredged, or picked up on the shore, and label- 

 led accordingly." — p. ix. 



In support of his view as to the second cause of error, Dr. 

 Gould gives various instances, shewing the difficulty which is 

 frequently experienced in the detection of specific differences, and 

 offers remarks deserving the anxious attention of conchologists. 

 He writes : — 



" When, therefore, we have before us shells from widely diverse 

 regions, apparently identical, they should be subjected to the 

 most careful scrutiny for structural differences. If no obvious 

 ones are detected, we may not consider the question as settled, 

 unless the animals have been compared; and we may go even 

 further, and require that their internal structure, as well as ex- 

 ternal features, should be examined. The number of instances 

 where this apparent ubiquity obtains is fast diminishing, as in 

 the cases already mentioned, in those of Cyprea exanthema, Cer- 

 vina and Cervinetta, &c. A large proportion of the shells inhab- 

 iting the eastern and western shores of the Atlantic, have been 

 regarded as identical ; and many of them are really so. But the 

 closer the comparison, the more it tends to diminish rather than 

 increase the identical species. The same is found true in regard 

 to other classes of animals. In fact the doctrine of the local lim- 

 itation of animals, even now, meets with so few apparent excep- 

 tions, that we admit it as an axiom in zoology, that species 

 strongly resembling each other, derived from widely diverse 

 localities, especially if a continent intervenes, and if no known 

 or plausible means of communication can be assigned, should he 

 assumed as different^ until their identity can be proved.* Much 

 study of living specimens must be had before the apparent excep- 

 tions can be brought under the rule." — p. x. 



* Collectors, whose attention has not been directed to geographical distribution, 

 are frequently misled, and may unconsciously mislead others, by their habit of label- 

 ing a shell found in the province in which they reside, with the identical name given 

 to a shell from a totally different province, because it agrees generally with the 

 figm-e and description met with in some conchological work, perliaps itself not par- 

 ticularly distinguished for accuracy as regards habitat. 



