258 THE CHAIN OP^ LIFE. 



in the Coal formation the only known Pulmonates, four or 

 five in number, belong to as many generic types. 



I have already referred to the permanence of certain species 

 in geological time. I may now place this in connection with 

 the law of origination and more or less continuous transmission 

 of varietal forms. I may, perhaps, best illustrate this in con- 

 nection with a group of species with which I am very familiar, 

 that which came into our seas at the !)eginning of the Olacial 

 age, and still exists. With regard to their permanence, it can 

 be affirmed that the shells now elevated in Wales to 1,200 and 

 in Canada to 600 feet above the sea, and which lived before 

 the last great revolution of our continents, a period vastly 

 remote as compared with human history, differ in no tittle from 

 their modern successors after thousands or tens of thousands 

 of generations. It can also be affirmed that the more variable 

 species api>ear under precisely the same varietal forms then as 

 now, though these varieties have changed much in their local 

 distribution. I'he real import of these statements, which might 

 also be made with regard to other groups well known to palae- 

 ontologists, is of so great significance that it can be rea.lised 

 only after we have thought of the vast time and numerous 

 changes through which these humble creatures have survived. 

 I may call in evidence here a familiar British and American 

 animal, the common sand clam, Mya aufmria, and its relative, 

 Mya truHcaia, which now inhabit together all the northern 

 seas; for the Pacific specimens, from Japan and California? 

 though differently named, are undoubtedly the same. Mya 

 truncata appears in Europe in the older Pliocene, and was 

 followed by M. arenaria a little later. Both shells occur in 

 the Pleistocene of America, and their several varietal forms 

 had then already developed themselves, and remain the 

 same to-day ; so that these humble mollusks, littoral in their 

 habits, and subjected to a great variety of conditions, have 

 continued, perhaps for one or two thousand centuries, to con- 

 struct their shells precisely as at present. Nor are there any 



