l8o THE SUCCESSION OF ANIMAL FORMS 



types, while the Myriapods and Amphibians alike appear in a 

 crowd of generic forms. 



I have already referred to the permanence of species in 

 geological time. We may now place this in connection with 

 the law of rapid origination and more or less continuous 

 transmission of varietal forms. A good illustration will be 

 afforded by a group of species with which I am very familiar, 

 that which came into our seas at the beginning of the Glacial 

 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 be- 

 fore the last great revolution of our continents a period very 

 remote as compared with human history differ in no tittle 

 from their modern successors after hundreds or thousands of 

 generations. It can also be affirmed that the more variable 

 species appear under precisely the same varietal forms then as 

 now, though these varieties have changed much in their local 

 distribution. The 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 realized 

 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 New England animal, 

 the common sand clam, Mya arenaria^ and its relative Mya 

 truncata, the short sand clam, which how 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 Coralline Crag, and 

 was followed by M. arenaria in the Red Crag. Both shells 

 occur in the Pleistocene of America, and their several varietal 

 forms had already developed themselves in the Crag, and re- 

 main the same to-day ; so that these humble mollusks, littoral 

 in their habits, and subjected to a great variety of conditions, 

 have continued for a very long period to construct their shells 



