INFLUENCE OF EXTERNAL AGENTS. 503 



furnishes it. Thus, in the development of a mammal, the first and lim- 

 ited supply of oxygen is from the portion contained in the liquids of the 

 ovum ; a far more copious one, at a later period, is derived from the pla- 

 cental mechanism ; but these subordinate states eventually give place to 

 the direct respiration of the open atmospheric air. As this gradual march 

 in the evolution of the. respiratory function is going forward, it is attend- 

 ed by a corresponding development of all the animal capabilities. 



So, too, on the great scale with genera and species. In the impure at- 

 mosphere of the earliest geological times, it was not possible that energet- 

 ic respiration could be carried on either by aquatic or by aerial animals. 

 Both may be included in the remark, for it is demonstrable that, on ordi- 

 nary physical principles, there must ever be a correspondence between the 

 chemical constitution of the atmospheric air and the gas of respiration 

 dissolved in the sea, or other natural waters. Abundant geological evi- 

 dence is before us to the effect that the entire respiratory medium, both 

 atmospheric and aquatic, has passed through a gradual amelioration, the 

 percentage amount of its irrespirable elements declining, and that of its 

 oxygen correspondingly increasing. The removal of those prodigious 

 masses of carbon deposited as coal satisfactorily establishes this point ; 

 and, therefore, as far as that medium is concerned, there is a general re- 

 semblance between the conditions under which the entire animal series 

 and the single individual have been placed. 



We might include in these remarks the vegetable as well as the ani- 

 mal series ; for, as respects flowering plants, it is the special function 

 of their floral or reproductive apparatus to discharge at a particular epoch 

 the functions of an animal in taking oxygen from the air, and replacing 

 it by carbonic acid. There would, therefore, be no cause for surprise if, 

 in that ancient carbonated atmosphere, cryptogamic plants alone could 

 maintain themselves, and that the flowering tribes could only appear after 

 a due change in the aerial constitution, which also gave to hot-blooded 

 animals the opportunity of coming forth. That change, as we have said, 

 consisted essentially in the appearance of a great excess of oxygen gas. 

 Such a superficial examination of the question shows that there is a par- 

 allelism between the physical conditions under which the animal series, 

 in the lapse of countless centuries, has been placed, and those to which, 

 in the shorter period of its history, the developing individual is submit- 

 ted, at least as respects the respiratory function. But it is to be re- 

 membered that respiration is the prime function in the animal economy. 



As regards the influence of heat, it has been remarked in the preceding 

 chapter that, at the period of the first appearance of organic i n fl uen ce of 

 forms, there was not only a high, but likewise a uniform tern- heat - 

 perature all over the globe. The evidence establishing this is already 

 given ; but if thus, in what might be termed the infancy of the organic 



