CHAPTER XXII. 



AN OUTLINE OF THE GEOGRAPHICAL DISTRIBUTION OF MOLLUSCA. 



The Mollusca being for the most part marine, it does not enter 

 into the plan of this work to go into much detail as to their 

 distribution. The orders and families will, however, be passed 

 briefly in review, and all terrestrial and fresh-water groups 

 discussed in somewhat more detail ; with the object of showing 

 how far their distribution accords with that of the higher 

 animals, and to what extent the anomalies they present can be 

 explained by peculiarities of organisation and habits. If the 

 views advocated in our fifth chapter are correct, the regions 

 there marked out must apply to all classes of animals ; and it 

 will be the task of the students of each group, to work out in 

 detail the causes which have led to any special features of 

 distribution. All I can hope to do here, is to show, generally 

 and tentatively, that such a mode of treatment is possible ; and 

 that it is not necessary, as it is certainly not convenient or 

 instructive, to have a distinct set of " Regions " established for 

 each class or order in the Animal and Vegetable Kingdoms. 



For all the Marine groups I have merely summarised the 

 information contained in Mr. Woodward's Manual of the 

 Mollusca, but in the case of the J,and Shells I have consulted 

 the most recent general works, and endeavoured to give an 

 accurate, though doubtless a very incomplete, account of the 

 most interesting facts in their distribution. As their classifica- 

 tion is very unsettled, I have followed that of the two latest 

 great works, by Martens and Pfeiffer. 



