28 PICTURESQUE SKETCHES 



tant changes must have since taken place in the con- 

 dition of the sea-bottom, but also explain the nature 

 of some of those changes. 



According to the actual constitution of things, the 

 soft substance of the bodies of animals consists chiefly 

 of carbon in combination with gases (oxygen, hydro- 

 gen, and nitrogen) ; and the more solid parts, whether 

 forming a bony skeleton, or a yet harder external 

 case, or internal framework of stone, are composed of 

 salts of lime with little admixture of other material, 

 especially in the invertebrated animals. The presence 

 of carbon, lime, and these gases, therefore, in sufficient 

 abundance under favourable circumstances of tempe- 

 rature and in a condition to combine with other ele- 

 ments, is all that is required to enable animals once 

 created to carry on the functions of life.* 



Of such substances, those existing in a gaseous 

 state were, no doubt, sufficiently abundant during 

 the earlier periods of the earth's development : car- 

 bon, of which so large a quantity is still given off in 

 volcanic districts in the form of carbonic acid gas, 

 must also have been abundant then ; and a sufficient 

 proportion of lime and silica is found in the compo- 

 sition of felspar one of the most important and uni- 

 versal ingredients of granite to supply the marine 

 animals with materials for their stony houses. It has, 

 indeed, been thought possible, though perhaps it is 



* It is indeed true, that at present most of these inorganic materials 

 are first prepared by vegetables into a fit pabulum for the animal ; but it 

 seems by no means certain that the lower animals cannot themselves per- 

 form the required change. Even if this is not the case, there is sufficient 

 proof in some of the most ancient fossiliferous rocks, that large masses of 

 sea-weed existed as soon as any other organized substance. 



