XCii INTRODUCTION. [CH. 



distinguished by Mr. Davidson (who has especially and so 

 thoroughly studied the fossil Brachiopoda) from a living 

 species (L. anatind) by any characters which he could 

 recognize as constituting a valid specific difference. 



These considerations, however, involve the difficult 

 question of the origin of species ; and I will not pursue 

 them further, except by suggesting the very great proba- 

 bility that all existing species have descended by modi- 

 fication from primeval forms, but at the same time not 

 admitting the hypothesis of Mr. Darwin that such forms 

 were very few or perhaps unique. In those strata which 

 contain our earliest records of the world's history, as 

 great a diversity of form is exhibited in the groups which 

 we call genera and species as in the existing fauna ; and 

 it seems evident that the plan of the Creator, so far as 

 we can comprehend it, has not been that of progressive 

 development. 



Nor will I here venture to touch upon the equally 

 abstruse, and more speculative, hypothesis as to the 

 radiation of species from several centres of creation. 



But I am digressing. For the reasons above stated 

 with regard to the connexion between the Coralline 

 Crag and British shells, I am inclined to regard this 

 formation as the starting-point, and as it were the 

 cradle of our molluscan race. The fauna of Europe, 

 Northern Asia, the Cis-Atlantic zone of Africa, and a 

 great part of North America appears to have been 

 closely related at a comparatively recent epoch, and 

 to constitute only one area of origin. Many species of 

 Mollusca once existed at both extremities of this vast 

 district. Mya truncata, Cyprina Islandica, and Bucci- 

 num undatum live in the Arctic and North Atlantic as 

 well as in our own seas, and their remains or shells 

 are found in Sicily. Cancellaria costellifera occurs^in 



