DR. DRAPER'S LECTURE ON EVOLUTION. 185 



Should the ices of the poles spread over the temperate region, the 

 reindeer would accompany their invading edge. 



While the environment thus influences the organism, the organism 

 reacting influences the environment. The most striking instance of 

 this, perhaps, will be found on comparing the constitution of the at- 

 mosphere before and since the Carboniferous epoch. Prior to that 

 epoch, all the myriads of tons of coaly substance now inclosed in the 

 strata of the earth existed as carbonic acid in the air. By the agen- 

 cy of the sunlight acting on the leaves of the luxuriant vegetation of 

 those times, this noxious gas was gradually removed, and replaced 

 by an equivalent volume of oxygen. A hot-blooded, quickly-respiring 

 animal could not possibly exist in an atmosphere laden with carbonic 

 acid. Anterior to the coal deposit, the fauna was cold-blooded and 

 slow-respiring. The flora thus changed the aerial environment, and 

 this, in its turn, reacting, changed the fauna. 



It is on all sides admitted that plants tend by their removal of 

 carbonic acid from the air, replacing it by oxygen, to compensate for 

 the disturbance occasioned by animals. In this way, through very 

 many centuries, the same percentage constitution of the atmosphere 

 is maintained, the sum total of vegetable being automatically ad- 

 justed to the sum total of animal life automatically, and not by any 

 interference of Providence a fact of great value in its connection 

 with the theory of evolution. For, if we admit what has been con- 

 clusively established by direct experiment, that plants would grow 

 more luxuriantly in an atmosphere somewhat richer in carbonic acid 

 than the existing one, we may see how upon this condition depends a 

 principle of conservation, which must forever retain the air at its 

 present constitution, no matter how animal life may vary. 



Cuvier speaks of the inferior organisms as furnishing us with a 

 series of experiments made by the hand of Nature, an idea often 

 quoted and often admired, but which, perhaps, is scarcely consistent 

 with enlarged conceptions of the system of the world. An organism, 

 no matter how high or low, is not in an attitude of isolation. It is 

 connected by intimate bonds with those above and those beneath. It 

 is no product of an experimental attempt, which, either on the part of 

 Nature or otherwise, has ended in failure or only partial success. 



The organic series an expression full of significance and full of 

 truth implies the interconnection of all organic forms the organic se- 

 ries is not the result of numberless creative blunders, abortive attempts 

 or freaks of Nature. It presents a far nobler aspect. Every member 

 of it, even the humblest plant, is perfect in itself. From a common 

 origin, or simple cell, all have arisen ; there is no perceptible micro- 

 scopic difference between the primordial vesicle which is to produce 

 the lowest plant, and that which is to produce the highest, but the 

 one, under the favoring circumstances to which it has been exposed, 



