DIVISIONS OF THE ANIMAL KINGDOM. XXXV11 



body is built up and sustained, and the respiratory organs 

 to the physical and chemical constitution of the elements 

 which the living creatures respire ; and when great changes 

 take place in the relations of living bodies with food, air, 

 and other external agents, either we -must suppose that the 

 species perish utterly, or that they become adapted to the 

 new conditions in which they are placed. The temperature 

 of this earth, and, consequently, of the air and water with 

 which it was in contact, must at one period have been ex- 

 ceedingly great, as measured by the sensations of animals 

 now living ; and with the temperature, the physical and che- 

 mical relations of the solids and fluids of the globe must 

 have varied. We cannot suppose that the pristine ocean 

 contained the same earthy, saline, and other constituents, in 

 the same proportions as the present seas, or that the at- 

 mosphere, with respect to density and other conditions, was 

 the same as now. But variations in the conditions of ex- 

 ternal nature, having taken place from era to era, we 

 have equal reason, at least, to believe that corresponding 

 changes have taken place in the form and attributes of 

 species, as that alternate destruction and creation have been 

 the law of nature. For what periods of time the condi- 

 tions of the earth, with its waters and surrounding gases. 

 have changed so little as to have remained suited to the 

 maintenance of existing species, we do not know ; but the 

 period must be believed to have been vastly great, when 

 measured by our ordinary conceptions of duration, though 

 but as a drop, perhaps, in the stream, when compared with 

 the whole duration of the period since animal life was called 

 forth upon our planet. The age of the gigantic mastodons, 

 the huge tapirs, and the extinct carnivora of the tertiary de- 

 posites, which must have long preceded the era of man, is 

 yet but as yesterday compared with the age of the great 

 reptiles of the lias and oolite ; and the age of these again 

 must have been inconceivably posterior to the era of the 

 fishes and mollusea of the first fossiliferous strata. Although, 



