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 atmosphere 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 agency 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 aërial 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 adjusted 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 conclusively 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 series 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 microscopic 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,