To this law the living being is no exception. The creatures of this world (and it is of such we are now speaking) are certainly not creators of energy; but in respect of the great law of the conservation of energy, such beings must be regarded in the very same light as any other machines.
But there is yet another analogy between living beings and inanimate machines. When we study the working of any machine, we find that each transformation of energy brought about has a material antecedent; the effect produced has a cause from which it springs, and this cause is one which we are probably able to recognise from our knowledge of the laws of matter. To take an example: in a steam-engine the amount of work produced depends upon the amount of heat carried from the boiler to the condenser; and this amount depends in its turn upon the amount of coal which is burned in the furnace of the engine. In like manner, the velocity of the bullet which issues from a rifle depends upon the transformation of the energy of the powder; this in its turn depends upon the explosion of the percussion cap; this again upon the fall of the trigger; and lastly this upon the finger of the man who fires the rifle.
Now, without attempting to define what life is, and leaving all speculations regarding it to our [last chapter], we yet think it may safely be said that a living being is analogous to a machine in this particular also.
Let us take the man who fires the rifle. We can trace back the motion of his forefinger to the contraction of a muscle; and we can go even further back and connect this contraction with a stimulus sent along the nerves from the brain, so that a material effect is here seen to be brought about by a material antecedent, just as truly as in an inanimate machine. Indeed, we may generalise, and say that, so far as we can physically investigate a living being, we may take it for granted that a material effect is due to a strictly material antecedent in his case also.
181. We have thus discussed two respects in which a living being is analogous to a machine, and the next point is to determine which of the two classes of machines most resembles the living being. Is he analogous to the solar system, a steam-engine, or a clock? or is he rather analogous to some delicately constructed machine, such, for instance, as a rifle? There can, we think, be no doubt that a living being most resembles a delicately constructed machine. For what is the characteristic of such a machine? It is that in it a comparatively great transformation of energy may be brought about by a comparatively small physical antecedent. Thus a slight breath of air may determine the fall of the egg off the table, or a slight tap the explosion of a large quantity of fulminating silver. So in the human being, a very small and obscure transmutation of energy in the mysterious brain-chamber may determine some very violent motion. ‘Life is not a bully who swaggers out into the open universe, upsetting the laws of energy in all directions, but rather a consummate strategist, who, sitting in his secret chamber over his wires, directs the movements of a great army.’[55]
182. Granting then that a living being is a delicately constructed machine, the next point is to determine what process of delicacy, what peculiar arrangement of unstable forces, is employed in his construction? Now it is very easy to perceive that the delicacy in this case is brought about by an unstable arrangement of chemical forces. It is plain that the body of an animal is a chemically unstable product, and if, as one consequence of this, great freedom of action and delicacy are possessed during life, it is another consequence that the extinction of life is very speedily followed by decay.
The body then owes its delicacy to its chemically unstable nature; to a peculiar collocation of particles which certainly would not, in virtue of their own merely physical forces, have united themselves together as we find them in the body.
183. To what, then, is due this peculiar grouping of particles in the living body?
We reply that it is, in one sense at least, derived from the food which is eaten. If animal food is eaten, it is of course derived from the body of the animal which is consumed. That animal may possibly have derived it from another animal, but more probably it has been derived in this case direct from the vegetable world. Ultimately, therefore, it is to this world that we must look as the source of that delicately constructed substance which plays such a wonderful and important part in the animal economy. If we go one link further back in the chain of causation, we shall be carried from the vegetable world to the sun as the great and ultimate physical source of that high-class energy and delicacy of construction which characterise vegetable products. It is, in truth, owing to the actinic rays of our luminary that vegetable tissue is manufactured in the leaves of plants, the carbonic acid of the air being decomposed, and oxygen given out, while the carbon, united with other substances, and modified thereby, is retained by the plant to form part of its substance, or perchance to become the food of animals.
184. We have therefore now arrived at the conclusion that the delicacy of construction which our frames require is ultimately derived from the sun, so far at least as the visible universe is concerned. If then we would reply to the question of this chapter, whether or not there may be beings superior to man connected with this present universe, let us look abroad and endeavour to ascertain whether there be in this universe any other obvious process of delicacy besides that which characterises the bodies of animals like ourselves.