S6 Professor Owen on British Fossil Bep tiles. 



And, besides the probability of such a condition of the zoo- 

 logical series being connected with the chemical modifications 

 of the air, the terrestrial reptiles, from the inferior energy of 

 their muscular contractions, and still more from the greater 

 irritability of the fibres, and power of continuing their actions, 

 would constitute the highest organized species, best adapted 

 to exist under greater atmospheric pressure than operates on 

 the surface of the earth at the present time. 



Through such a medium, approaching in a corresponding de- 

 gree to the physical properties of water, a cold-blooded animal 

 might even rise above the surface and wing its heavy flight, 

 since this would demand less energetic muscular actions than 

 are now requisite for such a kind of locomotion ; and thus we 

 may conceive why the atmosphere of our planet, during the ear- 

 lier oolitic periods, may have been traversed by creatures of no 

 higher organization than Saurians. If we may presume to 

 conjecture that atmospheric pressure has been diminished by 

 a change in the composition as well as by a diminution of the 

 general mass of the air, the beautiful adaptation of the struc- 

 ture of birds to a medium thus rendered both lighter and more 

 invigorating, by the abstraction of carbon and an increase of 

 oxygen, must be appreciable by every physiologist. And it is 

 not without interest to observe, that the period when such a 

 change would be thus indicated by the first appearance of birds 

 in the Wealden strata,* is likewise characterized by the pre- 

 valence of those dinosaurian reptiles which in structure most 



* Foot-prints alone, like those termed ^^ Ornithichnites," observed in the 

 new red sandstone of Connecticut, are insufficient to support the inference 

 of the possession of the highly developed organization of a bird of flight by the 

 creatures which have left them. The Rhynchosaur and biped Pterodactyles 

 already warn us how closely the Ornithic type may be approached without 

 the essential characters of the Saurian being lost. By the Chirotherian Ich- 

 nolites vve learn how closely an animal, in all probability a Batrachian, may 

 resemble a pedimanous mammal in the form of its foot-prints. 



The degree in which flying insects can resist noxious gases, which would 

 be quickly fatal to the warm-blooded vertebrates, invalidates the objection to 

 a progressive change of atmosphere having accompanied the prevalence of 

 quick breathing animals, which might be suggested by the LibeUuI(t of the 

 liat and by the oolitic beetles. 



