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In darkness this process ceases. It ceases, too, when the quantities of lightand heat received are greatly reduced, as in winter. Conversely, it is activewhen the light and heat are great, as in summer. And the like relation isseen in the fact that while plant-life is luxuriant in the tropics, it diminishesin temperate regions, and disappears as we approach the poles. Thus the irresistibleinference is that the forces by which plants grow and carry on their functions,are forces which previously existed as solar radiations.

That in the main, the processes of animal life are opposite to those ofvegetal life is a truth long current among men of science. Chemically considered,vegetal life is chiefly a process of de-oxidation, and animal life chieflya process of oxidation; chiefly we must say, because in so far as plantsare expenders of force for the purposes of organization, they are oxidizers;and animals, in some of their minor processes, are probably de-oxidizers.

But with this qualification, the general truth is that while the plant, decomposingcarbon dioxide and water and liberating oxygen, builds up the detained carbonand hydrogen (along with a little nitrogen and small quantities of otherelements) into stem, branches, leaves, and seeds; the animal, consuming thesebranches, leaves, and seeds, and absorbing oxygen, re-composes carbon dioxideand water, forming also certain nitrogenous compounds in minor amounts. Andwhile the decomposition effected by the plant is at the expense of energiesemanating from the Sun, the re-composition effected by the animal is at theprofit of these energies, which are liberated during the combination of suchelements. Thus the movements, internal and external, of the animal, are re-appearancesin new forms of a power absorbed by the plant under the shape of light andheat. Just as the solar forces expended in raising vapour from the sea'ssurface, are given out again in the fall of rain and rivers to the same level,and in the accompanying transfer of solid matters; so, the solar forces thatin the plant raised certain chemical elements to a condition of unstableequilibrium, are given out again t the actions of the animal during the fallof these elements to a condition of stable equilibrium.

Besides thus tracing a qualitative correlation between these two greatorders of organic activity, as well as between both of them and inorganicactivities, we may rudely trace a quantitative correlation. Where vegetallife is abundant, we usually find abundant animal life; and as we advancefrom torrid to temperate and frigid climates, the two decrease together.

Speaking generally, the animals of each class reach larger sizes in regionswhere vegetation is luxuriant, than in those where it is sparse.

Certain facts of development in both plants and animals, illustrate stillmore directly the truth we are considering. In pursuance of a suggestionmade by Mr. (afterwards Sir William) Grove, Dr. Carpenter pointed out thata connexion between physical and vital forces is exhibited during incubation.

The transformation of the unorganized contents of an egg into the organizedchick, is a question of heat: withhold heat and the process does not commence;supply heat and it goes on while the temperature is maintained, but ceaseswhen the egg is allowed to cool. The developmental changes can be completedonly by keeping the temperature with tolerable constancy at a definite heightfor a definite time; that is -- only by supplying a definite quantity ofheat. Though the proclivities of the molecules determine the typical structureassumed, yet the energy supplied by the thermal undulations gives them thepower of arranging themselves into that structure. In the metamorphoses ofinsects we may discern parallel facts. The hatching of their eggs is determinedby temperature, as is also the evolution of the pupa into the imago; andboth are accelerated or retarded according as heat is artificially suppliedor withheld. It will suffice just to add, that the germination of plantspresents like relations of cause and effect, as every season shows .

Thus then the various changes exhibited by the organic creation, whetherconsidered as a whole, or in its two great divisions, or in its individualmembers, conform, so far as we can ascertain, to the general principle. §71. Even after all that has been said in the foregoing part of thiswork, many will be alarmed by the assertion that the forces which we distinguishas mental, come within the same generalization. Yet there is no alternativebut to make this assertion: the facts which justify, or rather which necessitate,it being abundant and conspicuous. At the same time they are extremely involved.

The essential correlations occur in organs which are mostly invisible, andbetween forces or energies quite other than those which are apparent. Letus first take a superficial view of the evidence.