78

Ontogenesis.

[LECT.

authority. The cells do this by grouping themselves into parties, which divide the labour of living by performing different functions. Some cells carry on the work of digestion. Some cells form the nerves which carry stimuli up to the brain; some cells make up the brain itself; some cells form the nerves which carry the messages of the brain to the muscles; while other cells compose the muscles whose duty it is to translate those messages into action. And all organisms, plants and animals alike, consist of a greater or smaller number of cells: in very low forms, such as an Amaba, there is only one cell which constitutes the creature; in the higher forms, such as man, there are countless millions.

Next, if you take any individual of the many-celled kind and trace its own separate life-history backwards, you will find that it began to exist as a single cell. The egg or germ in which a plant or an animal begins, in general, its individual life, is a single cell, and the earliest stage of its development consists in this cell multiplying itself as an Amaba does, by splitting up into two cells. These two cells, however, remain together as component parts of the one animal or plant, but they nevertheless possess enough independent vitality to multiply themselves, in their turn, by splitting up each into two cells, making four in all. These again divide, and so the process goes on, the cells becoming more and more numerous and more and more various, their variety suiting them to the various kinds of work they have to do, in promoting the unity and welfare of the being whose parts they are. It is at least conceivable that any single complex organism is built up as the result of a process of natural selection in the struggle for life amongst individual cells, just as the sum of living beings on the earth's surface is, according to Darwin, the result of a similar process acting amongst individual organisms.

We have, then, two aspects of life development: the develop

II.]

66

Spontaneous Generation."

79

ment of different races from a common stock, and the development of an individual composed of many cells from a single cell. About this latter process there can be no doubt; naturalists tell us that it takes place under their very eyes. And we can easily understand that any one who has watched the marvellous changes by which an egg or germ grows into a highly organised animal, or a seed into a tree, will have little difficulty in accepting as possible and even very probable the theory which asserts that in the course of ages beings like Amaba have been gradually transformed into man. Nor need we wonder that the development of an animal from its primitive germ has been regarded as a brief repetition of the long process by which an ancestral race of beings like the germ were changed into the species to which the animal belongs; so that the story of the individual's early growth is a history in miniature of the evolution of its race.

Supposing then that we recognise the multitude of species now inhabiting the earth as descendants of some very simple race of beings, say even of a single germ, the question at once faces us,-Whence came that germ? And the answer which some naturalists, though by no means all, would give, is that it arose by what is awkwardly called spontaneous generation, out of common lifeless matter. This of course leads to the further question-Have we any experimental evidence to show that a living being is ever produced out of not-living matter? To this question the answer is decidedly "No;" in all cases where we have seen living beings produced, they are the descendants of other living beings. At first sight it might seem to be otherwise. If you take, say, a solution of sugar-not a living thing-and let it stand for a time, you will find it soon swarm with simple organisms. But it has been proved with great certainty that in all such cases the organisms really come from germs, carried by the air or by other means to the liquid in which life appears;

80

The Meteoric Transfer.

[LECT.

and if you take sufficient care to keep the germs of life out, the liquid will remain lifeless as long as you choose to preserve it. Of course, to say that we have no evidence that life ever comes except from life, is not the same thing as to assert that life never under any circumstances has come or can come except from life but I think I am right in saying that most naturalists incline to the belief that the doctrine omne vivum ex vivo is true throughout all space and all time.

Now we saw that some fifteen or it may be fifty millions of years ago the earth was too hot to permit of life on its surface. And when it cooled, if life did not originate on it by spontaneous generation, must we suppose an act of creation to have taken place? Not necessarily; for as Helmholtz and Thomson have suggested, the germs of life may have come to the earth from other globes, borne by those stray fragments which we know sometimes strike our planet. And indeed it is not impossible that meteoric stones may have more than once been the carriers of creatures, in different stages of development; so that we may to a certain extent imagine that the process of evolution of species, whose results we now witness, did not all happen on the surface of the earth in the comparatively short period during which terrestrial life has been possible, but took place partly throughout the wider theatre, during the far longer ages, and under the more various atmospheric conditions, which other worlds have doubtless afforded. Even taking this view, however, of the origin of terrestrial life, it is scarcely possible to suppose that living beings have existed in the visible universe for as long a time as dead matter.

When we attempt to conceive of the formation of a living being out of dead matter by any natural process, we are brought face to face with the grand problem,-What is life? Are its characteristics essentially different from the characteristics of matter, or can we suppose that a suitably arranged collection of

II.]

What is Life?

81

common molecules would possess the qualities of a living being? To put the question in other words: Is an animal a machine in the same sense in which a steam engine with its boiler is a machine ?

As a matter of definition it is, I believe, usual to call a thing living when it has three characteristics. It must feed itself; it must respond to stimuli; and it must possess, at least potentially, the capacity of producing others of its kind. Now we could certainly imitate any one or all of these actions by a sufficiently complex mechanism. We could make, or rather we could assert that a clever engineer, with plenty of materials, men, time and tools at his command, could make a machine which would kick when it was pricked, so to speak; which could stoke itself with energy, provided a supply of energy was put within its reach; and which could even go on turning out other machines like itself. In fact, the visible phenomena of vitality are conceivably nothing more than mechanical. Please do not suppose me to say that they are nothing more than mechanical: all we can say is that the actions which are performed by an Amoeba or by any higher organism, in response to any stimulus, are conceivably not different in kind from the actions which take place when a touch is given to the valve of an engine or to the contact-making key of a telegraph. And therefore the passage from not-living matter to a living being is thinkable, so far as the merely visible qualities of that being are concerned: I say it is thinkable, although we have no evidence to show that such a passage has ever occurred.

Now let us pursue this very important point a step farther. Suppose one of you were to run a pin into me: you would find that I possessed the property of irritability, which is one of the essential properties of a living being. For I should respond to your stimulus by giving a start and perhaps making a sound. Those actions, however, would not prove that I am essentially

82

Vitality perhaps mechanical;

[LECT. different from dead matter: they would only prove that I am possessed of a very complex structure. For you might fairly say that the mechanical disturbance which the prick produced caused those mechanical actions through a perfectly mechanical chain of sequence. You set a sensory nerve throbbing: its vibrations travelled up until they came to a point of communication with motor nerves. The message passed down these to the muscles, which consequently contracted, and movement and sound were the results. Of course you cannot rigorously trace the sequence, nor see exactly how each motion happens. as the dynamical consequence of those which precede it; but still, here is a series of events which perhaps follow each other in as strict a mechanical order as the movements of an engine follow the touching of its valve. So far then as you can judge from this pin-pricking experiment, you see nothing about me which cannot, possibly, be explained as matter and the motion of matter.

But I know better. I know that, besides all this train of physical events, there has been something else which is of a wholly different kind. I was conscious of your pin-prick. I felt pain.

And so we find that, in addition to the effects of the prick which were visible to you, there were others of which I alone was directly aware. Now the question is, are these latter-the facts of consciousness-explainable as matter or the motions of matter? Or, to put it more generally, are they in any possible way physical in the sense that matter and electricity and chemical actions and energy generally are physical? For if so, then clearly we should have no room for any other gospel than a gospel of matter, and the idea that the death of the body is not the end of the individual life would be on the face of it absurd. If we could express thought and feeling in terms of the things which physical science deals with, then the only possible philosophy for us would be materialism. From the clash of dead