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THE EUROPEAN AGE OF REASON-(Continued).
THE NATURE AND RELATIONS OF MAN.
HEN the ancient doctrine of the plurality of worlds was parent position of
restored by Bruno, Galileo, and other modern astronoheliocentric mers, the resistance it encountered was mainly owing to its theory. anticipated bearing on the nature and relations of man. It
was said, if round our sun, as a centre, there revolve so many planetary bodies, experiencing the changes of summer and winter, day and night-bcdies illuminated by satellites, and perhaps enjoying twilight and other benefits such as have been conferred on the earth-shall we not consider them the abodes of accountable, perhaps of sinful, beings like ourselves ? Nay more, if each of the innumerable fixed stars is, as our sun, a central focus of light, attended by dark and revolving globes, is it not necessary to admit that they also have their inhabitants? But among so many families of intelligent beings, how is it that we, the denizens of an insignificant speck, have alone been found worthy of God's regard ?
It was this reasoning that sustained the geocentric theory, and made the earth the centre of the universe, the most noble of created things; the sun, the moon, the stars being only
ministers for the service of man. The fallacy But, like many other objects urged in that memorable conof objec
flict, this was founded on a misconception, or rather, on imthat theory.
perfect knowledge. There may be an infinity of worlds placed under the mechanical relations alluded to, but there may not be one among them that can be the abode of life. The physi
Nature of Animals.
327 cal conditions under which organization is possible are so numerous and so strictly limited, that the chances are millions to one against their conjoined occurrence.
In a religious point of view, we are greatly indebted to Geo-Evidence logy for the light it has cast on this objection. It has taught by Geology. us that during inconceivable lapses of time our earth itself contained no living thing. These were those preorganic ages to which reference was made in the last chapter. Then, by slow degrees, as a possibility of existence occurred, there gradually emerged one type after another. It is as but yesterday that the life of man could be maintained. Only in the presence of special physical conditions can an
tory nature animal exist. Even then it is essentially ephemeral. The life of living
forms. of it, as a whole, depends on the death of its integrant parts. In a waterfall, which maintains its place and appearance unchanged for many years, the constituent portions that have been precipitated headlong glide finally and for ever away. For the transitory matter to exhibit a permanent form, it is necessary that there should be a perpetual supply and also a perpetual removal. So long as the jutting ledge over which the waters rush, and the broken gulf below that receives them, remain unchanged, the cataract presents the same appearance. But variations in them mould it into a new shape ; its colour changes with a clear or a cloudy sky; the rainbow seen in its spray disappears when the beams of the sun are withdrawn.
So in that collection of substance which constitutes an ani. mal; whatever may be its position, high or low, in the realm of life, there is a perpetual introduction of new material and a perpetual departure of the old. It is a form, rather than an individual, that we see. Its permanence altogether depends on the permanence of the external conditions. If they change, it also changes, and a new form is the result.
An animal is therefore a form through which material sub- Character. stance is visibly passing, and suffering transmutation into new animal life. products. In that act of transmutation force is disengaged. That which we call its life is the display of the manner in which the force thus disengaged is expended.
A scientific examination of animal life must include two
rived from the sun.
Matter and primary facts. It must consider whence and in what manner
the stream of material substance has been derived, in what manner and whither it passes away. And since force cannot be created from nothing, and is in its very nature indestructi. ble, it must determine from what source that which is displayed by animals has been obtained, in what manner it is employed,
and what disposal is made of it eventually. Force is de The force thus expended is originally derived from the sun.
Plants are the intermedium for its conveyance. The inorganic material of a saline nature entering into their constitution is obtained from the soil in which they grow, as is also, for the most part, the water they require ; but their organic substance is derived from the surrounding atmosphere, and hence it is
strictly true that they are condensations from the air. Mode in These statements may be sufficiently illustrated, and the which plants ob- relation between plants and animals shown, by tracing the terial sub. course of any one of the ingredients entering into the vegeta
ble composition, and derived, as has been said, from the air. For this purpose, if we select their chief solid element, carbon, the remarks applicable to the course it follows will hold good for other accompanying elements. It is scarcely necessary to embarrass the brief exposition of vegetable life now to be given by any historical details, since these will come with more propriety subsequently. It is sufficient to mention that the chemical explanations of vegetable physiology rest essentially on the discovery of oxygen gas by Priestley, of the constitution of carbonic acid by Lavoisier, and of water by Cavendish and
Watt. Action of a While the sun is shining, the green parts of plants, espeplant on the
cially the leaves, decompose carbonic acid, one of the ingredients of the atmospheric air. This substance is composed of two elements, carbon and oxygen ; the former is appropriated by the plant, and enters into the composition of elaborated or descending sap, from which forthwith organic products, such as starch, sugar, wood-fibre, acids, and bases, are made. The other element, the oxygen, is for the most part refused by the plant, and returns to the air. As the process of decomposition goes on, new portions of carbonic acid are pre
Relation of Plants to the Air.
329 sented through mechanical movements, the trembling of the leaf, breezes, and currents rising from the foliage warmed by the solar beams giving place to other cool currents that set in below.
The action of a plant upon the air is therefore the separation of combustible material from that medium. Carbon is thus obtained from carbonic acid ; hydrogen from water. Plant life is chemically an operation of reduction, for in like manner ammonia is decomposed into its constituents, which are nitrogen and hydrogen ; and sulphuric and phosphoric acids, which, like ammonia, may have been brought into the plant through its roots in the form of salt bodies, are made to yield up the oxygen with which they had been combined, and their sulphur and phosphorus, combustible elements, are appropriated.
Every plant, from the humblest moss to the oak of a thou- Composisand
years, is thus formed by the sun from material obtained solution of from the air,-combustible material once united with oxygen, force. but now separated from that body. It is of especial importance to remark that in this act of decomposition, force, under the form of light, has disappeared, and become incorporated with the combustible, the organizing material. This force is surrendered again, or reappears, whenever the converse operation, combination with oxygen, occurs.
Vegetable products thus constitute a magazine in which force is stored up and preserved for any assignable time. Hence they are adapted for animal food and for the procuring of warmth. The heat evolved in the combustion of coal in domestic economy was originally light from the sun appropriated by plants in the Secondary geological times, and locked up for
The sun is also the source from which was derived the light obtained in all our artificial operations of burning gas, oil, fat, wax, for the purposes of illumination. My own experiments have proved ("Physiology,' p. 461)
of physical that it is the light of the sun, in contradistinction to the heat, forces. which occasions the decomposition of carbonic acid, furnishing carbon to plants and oxygen to the atmosphere. But such is the relation of the so-called imponderable principles of chemistry to each other, and their mutual convertibility, that that
The nature of food.
which has disappeared in performing its function as light may reappear as heat or electricity, or in the production of some mechanical effect.
Food is used by all animals for the sake of the force it thus contains, the remark applying to the carnivora as well as the herbivora. In both cases the source of supply is the vegetable kingdom, indirectly or directly. The plant is thus indispensable to the animal. It is the collector and preserver of that force the expenditure of which constitutes the special display of animal life.
From this point of view, animals must therefore be considered as machines, in which force, obtained as has been described, is utilized. The food they take, or the tissue that has been formed from it, is acted upon by the air they breathe, and undergoes partial or total oxidation, and now emerges again, in part as heat, in part as nerve-force, in some few instances in part as light or electricity, the force that originally
came from the sun. Cycle There is, therefore, a cycle or revolution through which through which mat- material particles suitable for organization incessantly run.
At one moment they exist as inorganic combinations in the air or the soil, then as portions of plants, then as portions of animals, then they return to the air or soil again to renew their cycle of movement. The metamorphoses feigned by the poets of antiquity have hence a foundation in fact, and the vegetable and animal, the organic and inorganic worlds are indissolubly bound together. Plants are reducing, animals oxidizing, machines. Plants form, animals destroy.
Thus, by the light of the sun, the carbonic acid of the atmosphere is decomposed,—its oxygen is set free, its carbon furnished to plants. The products obtained serve for the food of animals, and in their systems the carbon is reoxidized by the air they respire, and, resuming the condition of carbonic acid, is thrown back into the atmosphere in the breath, ready to be decomposed by the sunlight once more, and run through the same cycle of changes again. The growth of a plant and the respiration of an animal are dependent on each other.
Material particles are thus the vehicles of force. They un