The gradual dispersion of certain Mollusks in Nexo 

 England, 



By Edward S. Morse. 



The rapid dispersion of Mollusks, as observed in certain 

 species, is of great interest in connection with the general 

 distribution of species from certain centres. 



The rapid invasion of hirge areas, by species not known 

 to have occurred there before, may account for the wide 

 distribution of species through certain geological horizons, 

 where their progenitors in earlier deposits are not known. 

 The sudden appearance of species throughout large geo- 

 logical areas has always been held as a strong point by 

 those who argue against the doctrine of derivation. 



In a long study of the Mollusca of New England for the 

 past twenty-four years, I have observed many changes 

 going on in the distribution of certain species which in- 

 dicates a much more rapid invasion of areas than had 

 before been supposed. Not only do we see this rapid 

 introduction of forms from other centres, but some 

 species of mollusks vary greatly in their relative scarcity 

 and abundance, when observed over considerable lapses 

 of time. 



In my little work entitled "Observations on the Terres- 

 trial Pulmonifera of Maine," forming the first part of 

 the Journal of the Portland Society of Natural History, 

 published in 1864, I commented on this change in the 

 relative scarcity and abundance of certain species as 

 compared with the observations made by Dr. J. W. 



(171) 



