Profes&or Owen on Britiifh Fossil Heptileif. 85 



the dilferences which comparative aiiatoiuy demonstrates to 

 have existed between the vertebrated inhabitants of the se- 

 condary epochs of the geological history of the earth, and the 

 tertiary and present periods, form legitimate grounds for spe- 

 culation, not only on the essential nature and causes of those 

 differences, but upon the progressive changes to which our 

 planet and its atmosphere may have been subject. For, whe- 

 ther there had been grounds for regarding the organic pheno- 

 mena of primeval times as earlier stages in the progressive 

 development and transmutation of species, or whether, as the 

 closest investigation of these phenomena seems to demonstrate, 

 they have been the result of expressly created and successively 

 introduced species, — they naturally lead the physiologist to 

 speculate on the varying conditions of the surrounding media 

 to which such organic differences may have related. 



Now, reptiles mainly and essentially differ from birds and 

 mammals in the less active performance of the respiratory 

 function, and in a lower and simpler structure of the lungs 

 and heart, whereby they become, so to say, less dependent on 

 the atmosphere or oxygen for existence. From their extra- 

 ordinary prevalence in the secondary periods, under varied 

 modifications of size and structure, severally adapting them to 

 the performance of those tasks in the economy of organic na- 

 ture which are now assigned to the warm-blooded and quick- 

 breathing classes, the physiologist is led to conjecture that the 

 atmosphere had not undergone those changes, which the con- 

 solidation and concentration of certain of its elements in sub- 

 sequent additions to the earth's crust may have occasioned, 

 during the long lapse of ages during which the extinction of 

 so large a proportion of the reptilian class took place. And 

 if the chemist, by wide and extended views of his science in 

 relation to geology and mineralogy, should demonstrate, as 

 the botanist, from considerations of the peculiai* features of 

 the extinct Flora, has been led to suspect, that the atmosphere 

 of this globe formerly contained more carbon and less oxygen 

 than at present, then the anatomist might, a priori^ have con- 

 cluded that the highest classes of animals suited to the respira- 

 tion of such a medium must have been the cold-blooded fishes 

 and reptiles. 



