Feathers tonh a ugVs Geological Report. 93 



where climate and food are not favorable, animals of the same 

 species may be expected to present a marked difference in 

 their external characters. The fine horse of Arabia, which is 

 cognate with the zebra of Africa, is a dwarf variety in Shet- 

 land, where climate and food have limited its stature and 

 even its functions so much that the mare only breeds once in 

 two years. This variety of external configuration is common 

 to the mollusca, which differ almost at every point of a coast, 

 as we see from the great variety of the forms of oysters ; the 

 same may be observed in the unio of fresh waters, where 

 those of the same kind affect the same kind of locality. If 

 such laws influence animals and plants now, we may reasonably 

 suppose them to have influenced them in geological times, under 

 similar circumstances. Of this general adaptation of causes we 

 have singular evidences in the tertiary group : the surface of 

 the earth being at length brought into a new and appropriate 

 state, we find quadrupeds, and of various kinds, beginning to 

 multiply, all of them, however, no doubt suited to the tempera- 

 ture, which appears from the fossil vegetables found in high lati- 

 tudes to have still had a general tropical character. We find 

 the elephants of our own period thus accommodated to par- 

 ticular regions, that of the arctic circle, as well as the rhino- 

 ceros, being prepared with a fleecy covering. But although 

 about one-fourth of the superficies of the globe has become 

 dry land, and that abundantly fitted for every class of terres- 

 trial animals known to us, yet most important races continue 

 to disappear, not the species only, for with the exception of 

 some found in the tertiary, all the species in whatever strata 

 found are extinct. The paloaotherea and the mastodon are ex- 

 tinct everywhere ; and the elephant, whose remains we find 

 scattered over this continent, is extinct here also. The mas- 

 todon was common to America, to Asia, and to Europe, and 



lew comparatively are in the pure argillaceous shales, which were probably the 

 ancient muddy bottoms of the waters. It follows from this reasoning that we may 

 expect to find, as we generally do, the same class of animals in the same strata. 



