438 



Miscellaneous. 



during the carboniferous period were again revived with the oolitic 

 system ; and, accordingly from the coal occurring in it, we draw 

 evidences of a removal of carbonic acid from the atmosphere, and a 

 supply of oxygen to it. But here also we are struck with the new 

 forms of animal life which have now sprung into existence : the 

 saurians which began to appear in the new red sandstone, have now 

 multiplied, and play on the shores of the oolitic land. Insects in- 

 habit the luxuriant forests which cover the land ; and that most 

 extraordinary of all created beings, the pterodactylus, or flying 

 lizard, executes the functions for which it was designed. The sea 

 has acquired new inhabitants ; not only monstrous reptiles, but new 

 forms of fishes, zoophyta, mollusca, and articulosa. But still we 

 find neither birds nor mammalia. The plants of this series afford 

 evidence sufficient of a tropical climate ; but the saurian animals 

 furnish proof yet more conclusive. You may remember the cause 

 of animal heat, as I stated it to you in a former lecture. It is a 

 combustion of certain unazotised ingredients of food by means of 

 the oxygen of the air ; and as a product of the combustion, carbonic 

 acid and water are formed. The heat occasioned by the conversion 

 of the carbon and hydrogen into carbonic acid and water in the 

 interior of the body, must be as great as if the elements were burned 

 in the open air. 



We mentioned that it was quite possible that marine animals may 

 have existed when terrestrial animals could not. Of course the 

 grand object of respiration is the same in both classes of animals, 

 viz. transformation of the food, and of particular constituents of the 

 blood, by means of the oxygen of the air. Hence aquatic respira- 

 tion differs from the aerial only in that water becomes the medium 

 of conveying the air to the respiratory organs. In the lower classes 

 of marine animals, the respiration is entirely cutaneous, the air not 

 being supplied through distinct channels, but by a transudation 

 through membranes permeable to it. As we go higher in the scale 

 we find a bronchial respiration, or respiration by means of lungs. 

 In these the water holding oxygen in solution meets with a net work 

 of veins, and aerates the blood circulating within them. The cause 

 of animal heat being a combustion of carbon and hydrogen by means 

 of oxygen, it is obvious that in the cold-blooded animals, the quan- 

 tity of oxygen required will be much less than in the warm-blooded. 

 Accordingly we fiud this to be the case. A tench lives for some time 

 in water containing only ^ its bulk of oxygen ; whilst river water 

 generally contains from § to 1 per cent, of this gas. Unfortunately 



