by Robert S. Ball.
Originally published in Longman's Magazine (Longman, Green, & Co.) vol.3 #13 (Nov 1883).
It was in the year 1831 that a naval expedition sailed from Devonport. That expedition consisted of a single vessel, Her Majesty's ship Beagle, a ten-gun brig under the command of Captain Fitzroy, R.N. The Beagle was a stout old wooden ship, destined on this occasion for a most pacific enterprise. Her duty was to survey parts of the coast of South America and some islands in the Pacific, and to carry a chain of chronometrical measurements round the world. Five years later the Beagle returned from this cruise, and thus brought to a close one of the most remarkable voyages that can be found in the annals of the British navy. Now, why was the cruise of the Beagle of such unparalleled importance? There have been many other surveying expeditions quite as successful. No doubt the memorable voyage of the Challenger accomplished much more surveying than the voyage of the Beagle. But we are gradually learning that even such achievements as those of the Challenger must sink into insignificance when compared with the voyage of the Beagle. I would rather liken the voyage of the Beagle to the immortal voyage of Columbus. In each case a new world was discovered.
When the voyage of the Beagle was planned, the captain expressed a wish that some scientific observer should join the expedition. A young naturalist, eager to see the glories of the tropics, volunteered his services and was accepted. He sailed in the ship. For the whole five years he diligently sought every opportunity to gain a knowledge of nature. He pondered on that knowledge when he came home. He added to it by further observation and matured it by careful thought. After many years of labour and of thought the naturalist of the Beagle produced a book. The name of the book was the 'Origin of Species,' the name of the author was Charles Darwin.
The 'Origin of Species' appeared when I was a student in college, and I can recall at this day the intense delight with which I read it. I was an instantaneous convert to the new doctrines, and I have felt their influence so much during all my subsequent life that I have ventured to choose this subject as the one on which to address you this evening. And here let me hasten to anticipate an objection. It is in the domain of natural history that the great achievements of Darwin have been wrought. It might be urged that the discussion of such a subject lay within the province of biologists or of geologists, but could hardly be considered a legitimate enterprise for those whose studies led them in other directions. But this is a view from which I dissent. I cannot admit the 'Origin of Species' to be the exclusive property of biologists. In a more capacious view of the subject it will be seen that the great doctrine of Evolution is of the very loftiest significance, and soars far above the distinction between one science and another to which we are accustomed.
It is interesting to note the wondrous change that is taking place, I might almost say that has taken place, in the popular estimate of the Darwinian theory. It has been well said that a new theory has often to run through three different phases. In the first place, every one exclaims that the theory is not true; then it is urged that the theory is contrary to religion; and, lastly, that everybody knew it long ago. The great doctrine of natural selection promulgated by Darwin has run through these courses. At its first publication it was received with an outburst of incredulity among the unthinking part of the community. Every one recollects the denunciations it received and the ridicule which the new doctrine had to encounter. But the theory of Darwin has survived that stage. It has also survived the attacks of those who denounce the theory as contrary to religion. The truth inherent in the principles of Darwin has quietly brushed aside such opposition, and now we hear but little of it. The funeral of Darwin at Westminster Abbey must be regarded as marking a momentous epoch in the history of thought. That the great doctrine would some day be accepted was a necessary truth, but I do not think that any one who recollects the publication of the 'Origin of Species' could then have anticipated the enormous change in educated opinion which the next quarter of a century was to disclose. Still less likely would it have seemed that the whole nation would have so far acknowledged Darwin that with one accord they demanded that his remains should be interred in the national mausoleum.
Darwin has worked out one of the most splendid details in the history of the universe. His methods and his theory have intimate connections with other branches of science, and some of these it is our object to consider in this discourse. In particular I propose to sketch the position which the Darwinian theory occupies with reference to a celebrated branch of astronomical speculation.
The sun is hot and the sun is pouring forth heat. Now heat, we know, is capable of measurement; quantities of heat can be measured as accurately as tons of coal. The daily outflow of heat from the sun is as measurable a quantity as the daily outflow of gas from the gas-works. The total amount of heat which the sun pours forth cannot, it is true, be very accurately estimated by our present knowledge. All that we are here concerned to know is that it is of the most stupendous magnitude. Even the daily consumption of the sun's heat by the earth is enormous; but that is only a minute fraction, less, indeed, than the 2,000-millionth part of the total torrent which pours from the sun. Sir John Herschel gave an elegant illustration of the splendid extravagance of the sun's daily expenditure. Suppose, he said, that a cylindrical glacier of ice, 45 miles in diameter, were to be incessantly darted into the sun with the velocity of light, about 180,000 miles in a second, the entire of this ice would be continuously liquefied by the daily radiation of heat.
It is a momentous question to inquire what replenishes the heat of the sun, or whether the sun's heat is ever replenished at all. Mark the significant consequence which is at issue. If the sun be not replenished, then its heat must gradually wane. Various sources of replenishment have been suggested. It would be leaving my present subject too far on one side to attempt to discuss this subject in any detail, but I must briefly indicate the resolutions that have been proposed. Every one is acquainted with the pleasing phenomenon of shooting stars which dash into our air with a train of light and sparks. Every shooting star is thus a source of heat: the heat is produced by the friction of the air against the rapidly moving body. The shooting stars fall into the earth's atmosphere by thousands and by millions. It is believed that they fall into the sun in vastly greater numbers. They must rain in on the sun with a profusion corresponding to his vast surface, and with a velocity corresponding to his intense power of attraction. Each shooting star develops vastly more heat at its plunge into the sun than it would have done had it fallen upon the earth. The heat derived from all the shooting stars which fall upon the earth is utterly insignificant, but it may be that the heat from the torrent of shooting stars which rain in upon the sun is not insignificant. It may reach very large proportions. Some have, indeed, supposed that the influx of heat to the sun from the perennial showers of shooting stars is adequate to compensate for the loss of heat which the sun sustains by his incessant radiation. I do not believe that this view can be even approximately correct. No one will deny that the descent of meteors contributes some heat to the sun; but what we do deny is, that the quantity of heat thus acquired is at all comparable with the colossal daily expenditure. The whole question is to a great extent merely one of calculation. It can be shown that a certain quantity of meteors would suffice, but that quantity is enormously great. If the sun swallowed up every century a mighty host of meteors so numerous and so heavy that their collective mass was equal to that of our entire earth, then the view I am attempting to confute would be maintained. But a little consideration will show that the existence of so mighty a quantity of meteoric matter as this would require lies far beyond all reasonable probability. It should be remembered that the sun could only absorb each year a very small fraction of the total number of meteors that are roaming through the system. If, therefore, the meteors were as abundant as this supposition would require, the whole solar system would teem with them to an incredible extent. It therefore seems certain that the heat of the sun cannot be entirely sustained by the influx of meteoric matter.
If the sun were merely like a vast incandescent mass of stone or of metal, it would cool at the rate of 5° or 10° a year. A few thousand years would reduce it to such a degree that it would no longer be the source of light and of heat, which it certainly has been for thousands of years. At first sight it would seem as if the result at which we have arrived is paradoxical, but this is not really the case. I cannot now attempt to go fully into the matter. It may be sufficient to state generally that the sun is really parting with its heat, but that the rate at which heat is lost is affected by a special and very remarkable property. As the sun loses heat it contracts, and in the act of contraction heat is developed. The heat thus developed ekes out the sun's resources, so that the losses due to radiation are partly compensated. The result of the whole inquiry can be very easily stated, and it embraces a truth of which it is difficult to overestimate the importance. The sun possesses a certain quantity of heat or of energy, and that energy is being gradually wasted in the depths of space. It would not perhaps be true to say that the sun is at present actually falling in temperature. If the sun be actually gaseous, it may strangely enough be getting hotter instead of colder, so long as it remains gaseous; but, however we look at the question, there is one statement which admits of no doubtful interpretation—as the heat is radiated away, so the particles which form the sun's mass are drawn more and more closely together. The total mass of the sun—its weight as placed in a scale—cannot decrease, but the bulk which the sun occupies must decrease and is at this moment decreasing, and, so far as we know, will continue to decrease until the sun is one hard mass of matter benumbed with the cold of space.
It is true that the process of shrinking is very slow; it is so slow that we cannot measure with our telescopes the decrease in the sun's bulk, but we can calculate what the alteration in the sun's bulk must be in order to supply the daily radiation of heat. The change is but very small when we consider the present size of the sun. At the present moment the sun has a diameter of 860,000 miles. Each year this diameter decreases by about 220 feet; this decrease is always taking place; the process is never reversed; it is not periodic like so many other phenomena of nature; in time the result must become of overwhelming importance. The sun's career as a source of light and heat is ultimately doomed to extinction. It has been calculated that the sun cannot radiate enough heat to maintain life on the earth for a period of 10,000,000 years more.
I must not linger any longer on this subject, which would indeed require not one, but several lectures for adequate treatment. In particular I am obliged to pass by without discussion the remarkable theory lately put forward by Sir C.W. Siemens in the hope of retrieving the sun's reputation as a spendthrift. Coming from an authority of such justly-deserved repute, this theory has naturally attracted a great deal of attention. It is incumbent on me to mention it here, because, if this theory should be ultimately found to be true, the views previously entertained as to the dissipation of the sun's energy would require the most profound modification. I have given this theory the attention which anything coming from an author of such eminence must merit, but it has failed to convince me, and I still remain of the opinion usually held before its publication.
One hundred years ago the diameter of the sun was four miles greater than it is at present. One thousand years ago the diameter of the sun was forty miles greater than it is at present. Ten thousand years ago the diameter of the sun was 400 miles greater than it is now. The advent of man upon the earth took place no doubt a long time ago, but in the history of the earth the advent of man is a comparatively recent phenomenon. Yet it seems certain that when man first trod our planet, the diameter of the sun must have been many hundreds, perhaps many thousands, of miles greater than it is at present. We must not, however, overestimate the significance of this statement. The diameter of the sun is at present 860,000 miles, so that a diminution of 10,000 miles would be little more than the hundredth part of its diameter. If the diameter of the sun were to shrink to-morrow to the extent of 10,000 miles, the change would not be appreciable to common observation, though even a much smaller change would not elude delicate astronomical measurement. The world on which the primitive man trod was certainly illuminated by a larger sun than that which now shines upon us. It does not necessarily follow that the climates must have been much hotter then than now. The question of warmth depends upon other matters as well as sunbeams, so that we must be cautious in any inferences drawn in this way, nor are any such inferences needed for our present purpose.
But we must not stop in our retrospect at the epoch even of primeval man. We must go back earlier and earlier through the long ages of the geologists, and back again still further to the earliest epochs, when life first began to dawn on the earth. Still we find no reason to suppose that the law of the sun's decreasing heat is not still maintained, and thus, as far as our present knowledge goes, we are bound to suppose that the sun must have been larger and larger the further our retrospect extends. I do not say that the rate at which the sun changes its diameter was then the same as the four miles per century which is an approximation to its present rate. It is sufficient for our purpose that the sun is larger and larger the further we peer back into the remote abyss of the past. There was a time when the sun must have been twice as large as it is at present; it must once have been three times as large; it must once have been ten times as large. How long ago that was, no one can venture to say. It would be rash to attempt any estimate; but we cannot stop at the stage when the sun was even ten times as large as it is at present; the arguments we have used will still apply with equal, if not greater, force. And, looking back earlier still, there was a time when the sun was once swollen to such an extent that the mighty orbit of Neptune itself would be merely a girdle around the stupendous globe. At that time the sun must have been a gaseous mass of almost inconceivable tenuity. We are not to suppose that the earth and the other planets were solid bodies deeply buried in the vast bulk of the sun. It seems evident that the planets were gaseous masses in those ancient days and undistinguishable from the sun, which gave them birth.
We are now able to make an attempt to trace the history of the solar system, and to indicate the share which Darwin has had in the solution of the noble problem. We do not inquire how the original nebula came into being ; our history must commence with the actual existence of this nebula. There is, let it be confessed, a great deal of obscurity still clinging to the subject. Though we may be sure that the great nebula once existed, we cannot with much confidence trace out the method by which the planets were actually formed. It seems to be generally thought that the nebula must have been originally endowed with a certain rotation. This may be regarded as certain; indeed, it would be infinitely improbable that the nebula should not have had some rotation. As the nebula began to radiate heat, so it must have begun to contract; and as it began to contract, it began to rotate more rapidly. This is only the consequence of a well-known dynamical principle. But as the nebula spins more and more rapidly, the cohesion of its parts is lessened by centrifugal force. The moment at length arrives when the centrifugal force detaches a fragment of the nebula. The process of condensation still continues both in the fragment and in the central mass; the fragment changes from the gaseous state to the liquid, perhaps even from the liquid to the solid, and thus becomes a planet. Still the central mass condenses, and spins more and more rapidly, until a rupture again takes place and a second planet is produced. Again, and still again, the same process is repeated, until at length we recognise the central mass as our great and glorious sun, diminished by incessant contraction, though still vast and brilliantly hot. One of the lesser fragments which he cast off has consolidated into our earth, while other fragments, greater and smaller, have formed the rest of the host of planets. There are many features in the planets which seem to corroborate this view of their origin. They all revolve around the sun in the same direction; they all revolve on their own axes in the same direction, that direction being also coincident with the sun's rotation on its axis. Most astronomers are agreed that the history of the solar system has been something of the kind that I have ventured to describe. Astronomers were thus the first evolutionists; they had sketched out a majestic scheme of evolution for the whole solar system, and now they are rejoiced to find that the great Doctrine of Evolution has received an extension to the whole domain of organic life by the splendid genius of Darwin.
At its first separation from the shrinking central nebula, our earth was probably a mass of glowing gas, of incredibly greater volume than it is at present. Gradually the earth parted with its heat by radiation, and commenced to shrink also. The temperature was so high, that iron and other still more refractory substances were actually in a state of vapour, but, as the temperature fell, these substances could not remain in the gaseous form; they condensed first into liquids, these liquids coalesced into a vast central mass, and still that mass went on cooling until its surface, passing through the various stages of incandescence, sank at length to a temperature comparatively cool. Still the earth was swathed with a deep and dense mantle of air, charged with an enormous load of watery vapour; but, as the temperature of the surface gradually decreased, at length the watery vapours were condensed and descended to form the oceans with which our earth is so largely covered. At this point the functions of the astronomer are at an end; he has traced in outline the manufacture of the earth from the primeval nebula; he has accounted for its revolution round the sun, for its rotation on its axis; he has accounted for the shape of the earth and for its internal heat. His work being done, he now hands over the continuance of the history to the biologist.
The lifeless earth is the canvas on which has been drawn the noblest picture which modern science has produced. It is Darwin who has drawn this picture. He has shown that the evolution of the lifeless earth from the nebula is but the prelude to an organic evolution of still greater interest and complexity. He has taken up the history of the earth at the point where the astronomer left it, and he has made discoveries which have influenced thought and opinion more than any other discoveries which have been made for centuries. We here encounter a very celebrated difficulty. The theory of Darwin requires life to begin with, but how did that life originate? I need hardly remind you of the celebrated controversy which has taken place on this subject. It has been contended that life can never be produced except from life; but just as stoutly has the opposite view been maintained. Can it be possible that the wondrous and complex phenomena known as life are purely material? Can a particle of matter which consists only of a definite number of atoms of definite chemical composition manifest any of those characters which characterise life? Take as an extreme instance the brain of an ant, which is not larger than a quarter of a good-sized pin's head. It would require a volume to describe what we know of the powers of ants. Huber showed this long ago, and Sir John Lubbock has lately reminded us of it, while adding further discoveries of his own. I here quote Darwin's vivid description; but it is only right to add that many different species of ants are referred to, though included under the common designation: 'Ants certainly communicate information to each other, and several unite for the same work, or for games of play. They recognise their fellow-ants after months of absence, and feel sympathy for each other. They build great edifices, keep them clean, close the door in the evening, and post sentries. They make roads as well as tunnels under rivers, and temporary bridges over them by clinging together. They collect food for the community, and when an object too large for entrance is brought to the nest, they enlarge the door, and afterwards build it up again. They store up seeds of which they prevent the germination, and which if damp are brought to the surface to dry. They keep aphides and other insects as milch cows. They go out to battle in regular bands, and freely sacrifice their lives for the common weal. They emigrate according to a peculiar plan. They capture slaves. They move the eggs of their aphides, as well as their own eggs and cocoons, into warm parts of the nest, in order that they may be quickly hatched.'[2]
Well may Darwin speak of the brain of an ant as one of the most wondrous particles of matter in the world. We are apt to think that it is impossible for so minute a piece of matter to possess the necessary complexity required for the discharge of such elaborate functions. The microscope will no doubt show some details in the ant's brain, but these fall hopelessly short of revealing the refinement which the ant's brain must really have. The microscope is not adequate to show us the texture of matter. It has been one of the great discoveries of modern times to enable us to form some numerical estimate of the exquisite delicacy of the fabric which we know as inert matter. Water, or air, or iron may be divided and subdivided, but the process cannot be carried on indefinitely. There is a well-defined limit. We are even able to make some approximation to the number of molecules in a given mass of matter. Sir W. Thomson has estimated that the number of atoms in a cubic inch of air is to be expressed by the number 3, followed by no fewer than twenty ciphers. The brain of the ant doubtless contains more atoms than an equal volume of air; but even if we suppose them to be the same, and if we take the size of an ant's brain to be a little globe one-thousandth of an inch in diameter, we are able to form some estimate of the number of atoms it must contain. The number is to be expressed by writing down 6, and following it by eleven ciphers. We can imagine these atoms grouped in so many various ways that even the complexity of the ant's brain may be intelligible when we have so many units to deal with. An illustration will perhaps make the argument clearer. Take a million and a half of little black marks, put them in a certain order, and we have a wondrous result—Darwin's 'Descent of Man.' This book merely consists of about a million and a half letters, placed one after the other in a certain order. Whatever be the complexity of the ant's brain, it is still hard to believe that it could not be fully described in 400,000 volumes, each as large as Darwin's work. Yet the number of molecules in the ant's brain is at least 400,000 times as great as the number of letters in the memorable volume in question.
It would seem that by merely studying the behaviour of an infusion of hay or a tincture of turnips in a test tube, we do not rise to the full magnificence of the problem as to whether life can have originated on the globe from the particles of inorganic matter.
Unusual, indeed, must be the circumstances which will have brought about such a combination of atoms as to form the first organic being. But great events are always unusual. Because we cannot repeatedly make an organised being from inert matter in our test tubes, are we to say that such an event can never once have occurred with the infinite opportunities of nature? We have in nature the most varied conditions of temperature, of pressure, and of chemical composition. Every corner of the earth and of the ocean has been the laboratory in which these experiments have been carried on. It is not necessary to suppose that such an event as the formation of an organised being shall have occurred often. If in the whole course of millions of years past it has once happened, either on the land or in the depths of the ocean, that a group of atoms, few or many, have been so segregated as to have the power of assimilating outside material, and the power of producing other groups more or less similar to themselves, then we have no more demands to make on the 'Theory of Spontaneous Generation.' The more we study the actual nature of matter the less improbable will it seem that organic beings should have so originated. One of the most obvious contrasts between organic and inorganic bodies seems to be the power of motion often inherent in the organised body, which is not possessed by the inorganic body; but this is really a superficial view of the question. Take any mass of inorganic matter, a drop of water or a grain of sand. Each of these bodies is composed of a certain number of ultimate atoms. We have no hope that we shall ever have a microscope sufficiently powerful to detect these atoms; but we nevertheless know that they exist, and we know several of their properties. We know, for instance, that even in solid bodies these particles are not at rest, that they are in rapid and ceaseless motion, even though the body may be as rigid as a diamond. In ultimate analysis we see that the atoms of inorganic matter seem to have that mobility which is frequently noticed as a characteristic of vital action. A mere rearrangement of the movements of the atoms of a grain of sand could confer on the grain of sand some of the attributes of an organised body.
The method Darwin adopted is of the most captivating simplicity. It is doubtless well known to many here, and I shall glance at it but very briefly. When the history of Science in our century comes to be written, the interest will culminate in the supreme discovery of Natural Selection.
There are so many modifying circumstances to be taken into account that it is not often easy to trace the actual course of natural selection ;but the leading idea is so simple that, once it is properly stated, I do not see how any reasonable person can refuse his assent. There is a well-known proverb, 'as like as two peas,' and at a superficial glance two peas are no doubt very like each other. They are like in their size, shape, and colour; they are like in their internal structure; but, when we look closely into the subject, no two peas are exactly alike. Take any two peas from a sack, and after a brief examination you will detect innumerable points of difference. Weighed in a careful balance, they have not the same actual weight; gauged with a pair of callipers, they have not the same size; and these differences extend to every minute part of the structure. One pea will have more nourishment stored up for the benefit of the future plant. Another will be better able to resist hurtful influences. That two peas should be so absolutely identical in every feature as to be indistinguishable is an impossibility, or, as a mathematician would say, the chances are infinitely against such an occurrence; and when the chances are such we may for all practical purposes consider them as non-existent. If we find that two peas are never really alike, neither shall we find that two organisms of any kind are really alike when attention is directed to minute points of distinction. A shepherd will laugh to scorn the idea that any two of his flock are so like that they could be mistaken. Even his dog knows better than that. A poultry fancier will see in his pets conspicuous marks of difference which are barely apparent to the unskilled eye. I need not multiply illustrations, which will occur to everybody; the innumerable variety of roses and of geraniums, of apples and of other fruits, will show how universal is the law of variety among all the productions of the organic world.
The great doctrine of natural selection is founded upon this susceptibility to variation. Suppose that you wished to improve the peas in your garden, it is quite possible to do so in a few years in the following manner: Take 100 peas, sow them and preserve the seed. You will have some thousands of seeds, but no two peas will be exactly alike; pick out the hundred heaviest seeds and sow them again next season. You will have a crop of thousands, from which you are again to pick out the heaviest hundred. As this process is repeated year by year you will find that within certain limits the peas are gradually increased in size from one generation to another, and thus it is that improved varieties can be artificially established. The success of this process depends merely upon taking judicious advantage of the variability inherent in the organic world. This we may call an artificial selection as opposed to the natural selection.
What we have here described as being produced artificially in the pea is going on everywhere on the grandest scale in nature. Take an illustration this time from animal life; and I choose, as one of the most widely known instances, some incidents in the history of the common herring, which exists in such countless myriads in our oceans. Those who frequent the sea are well acquainted with certain features in the life of the herring. The herring is a fish deservedly prized for food, but it is not only mankind that are fond of devouring the herring; a similar taste is widely spread among the fowls of the air and the fishes of the sea. The herring has no defence from innumerable enemies but his agility and his caution. Around the shoal swarm troops of porpoises, while pollock and various other predatory fish follow the shoal wherever they go and devour the herrings in countless myriads. The female herring lays a stupendous quantity of eggs. It is perfectly certain that only a very minute fraction of these eggs ever reach maturity. If only one per cent. of the eggs grew to full size and reproduced more herrings, the herring population of the sea must increase manifold every year. This cannot always, or indeed often, be the case, and we are thus compelled to believe that out of every million herring eggs only a small fraction usually come to maturity. To those who have ever observed the herring this appalling mortality will not seem strange. To begin with, when the herring eggs are laid the flat fishes congregate and feast on the eggs to such an extent that fishermen repair to these spots and catch the flat fish in scores with their stomachs filled with the eggs of the herring. No doubt there are many other enemies at this stage, so that vast multitudes of the herring eggs never become hatched at all; even those that are hatched have indeed an anxious time of it. Around our coast we see in the autumn shoals of the tiny herrings pursued and devoured by hosts of young codfish and mackerel. Sometimes the fish surround the shoal completely, and the miserable prey cluster together near the top of the water in a vain hope of safety; but, alas! here the enemies from the air attack them. Sea-gulls crowd to the spot, gannets swallow the young herrings in mouthfuls, the rolling of porpoises adds more life to the scene, and once a shoal has been thus imprisoned between air and water, the slaughter is truly prodigious. The voracity of enemies is not the only danger to which young herrings are exposed; often they are left on the beach by the falling tide, and may be seen lying in hundreds along the sea margin. I purposely leave out of account all mere human enemies. The efforts of man in catching herrings are quite insignificant in comparison with their more numerous and incessantly voracious destroyers. Indeed Professor Huxley states that the codfish caught in our seas each season would, if they had not been caught, have eaten as many herrings during the next season as those which have actually fallen to the nets of the fisherman. The survivors of this fearful massacre are naturally objects of very great interest. How is it that they have been spared when so many myriads of their brothers and sisters have been annihilated. No doubt their safety is partly due to the chapter of accidents. They happen to be out of the way when the mackerel made a fatal rush. The sea-gull had eaten so many that when it came to their turn he positively could not eat any more. They got into the middle of the shoal afterwards and escaped the fish that preyed on its margin. But, making every allowance for the benefit of the accidents, I think we must credit the surviving herrings themselves with some share in their success. The few that have survived were certainly not the most stupid. They must have had quick sight, they must have had nimble fins, they must have had vigilance and activity. They must have been skilful in procuring food as well as alert in avoiding danger. They had no maternal solicitude to watch over them. Every little herring had to forage for himself, and to hide from or elude his enemies as well as he could; he had no kind warning that the tide was falling and that he would be left high and dry if he did not keep away from the edge. I think we must admit that the few herrings that survive out of a million eggs are above the average in whatever qualities constitute excellence in a herring. I will not say that they must be actually the very best, but I think we must admit that they were among the best.
What we have here attempted to illustrate takes place in the whole realm of organised life. The organic beings, animal and vegetable, tend to increase faster than the food or the presence of enemies will permit. Many must therefore perish. No two of these organisms are exactly identical. There will be trifling differences (sometimes, indeed, the differences are by no means trifling). It thus happens that in the struggle for life one individual will have a slight advantage over another. It therefore may be anticipated that the more favoured individuals will be those which survive; their peculiarities will be more or less inherited by their descendants. Thus the variations which are useful to the animal will in successive generations be gradually added to, and in course of time the widest changes in organisation can thus arise.
It may at first seem hard to realise that so trifling a change as that between one generation and the next can ever by repetition amount up to so great a change as that between one species of animal and another; still less can we imagine at first how animals so widely distinct as, for instance, a bird and a fish, can have originated by natural selection from some common ancestor. The whole question is chiefly one of time, complicated, it must be admitted, by many details; but it is easy to show how minute differences between one generation and the next, all tending in one direction, speedily reach to an appreciable amount. Let me give an illustration. I know some tender mothers who like to have their darlings photographed every year in order to preserve a permanent record of their development. No doubt the mother would have no difficulty in distinguishing between the photographs of her child at two years old or at three, or even between those of her boy at thirteen and at fourteen. But suppose that, instead of having the child photographed only once a year, he were to be photographed every week from birth until he was full grown. This is not at all an impracticable suggestion; there would be little more than a thousand photographs altogether. An album could easily be made which would hold them all. Of course the prudent mother would mark the dates on the back; but suppose this was not done, and the whole thousand photographs got into confusion, would it be possible to arrange them all in order again? Certainly no outsider could do it; he could sort them in a general way, so as to have the babies at one end, and the young men at the other, and the boys in the middle. But could he put the whole thousand in regular order from one end to the other? He could not. I doubt very much whether even the mother herself could do it without numerous faults. Now, if this be granted, the great difficulty in believing natural selection to be the origin of species will be lessened. Great as is the difference between a newborn infant and a man of twenty, the one passes into the other by such imperceptible gradations that the boy of this Monday is hardly distinguishable from the boy of last Monday or of next Monday. We thus see that if we divide the growth of an individual man into one thousand stages the passage from one stage to the next is almost imperceptible. In the same way, if we subdivide the growth of a species into a thousand parts or a million parts, we shall have gradations quite comparable with those we meet with in the ordinary variation from one generation to the next.
Nor is it hard to see how the process of natural selection has gradually produced diverging branches from the parent stem. The variations which occur may be of use to the organism in various ways. Among the progeny of a single pair there may be two individuals, A and B, which are specially favoured; but they may be favoured in different ways. A may have some increased facility in catching his prey; B, by his peculiar colour, of greater activity, may have superior power of eluding his enemies. The descendants of A will gradually from one generation to the next strengthen and reinforce the special feature which characterised A. The descendants of B will grow more and more adapted for eluding their enemies. The influence of natural selection is in both cases promoting the survival of the fittest, but each generation will see the cousins more and more widely separated. In no case indeed would the process be so simple as that here described—a multitude of circumstances will occur to complicate it; but enough has been said to show that in the great principle of natural selection we have a means of producing animals and plants which in the course of time will differ widely from other organisms from the same progenitors.
No one has ever seen a new species developed by natural selection; but this is because no one has ever lived long enough for that purpose. The circumstantial evidence in favour of natural selection is indeed so strong that no unprejudiced person can refuse to accept it. That evidence has of late years been poured out with a profusion which could hardly have been anticipated at the time when the 'Origin of Species' was published. Entombed within solid rocks we find fossil remains of the former inhabitants of our earth. There lies in these rocks a record of vast extent and of the most supreme interest, but that record is to a great extent screened from our view. Here and there fossils have been brought to light; but the greater part of the earth has never been examined, and we have as yet only the veriest fragments of the geological record before us. But these fragments of the record are of the most intense importance; they show us several of the links which connect one class of animals with another in the way the Darwinian theory suggests; and they encourage us to hope that, when the geological record shall have been fully explored, we shall have glimpses of a majestic panorama of the salient points in the history of life on our globe.
Mathematicians are long accustomed to the use of what is known as the infinitesimal calculus. It is indeed chiefly the infinitesimal calculus which has raised the science of mathematics to its present position, and which has given to that science a potent grasp over some of the inmost recesses of nature. Suppose, for instance, to take one of the most profound problems, we proceed to investigate on mathematical principles the movement of one of the planets. The sun, in the first place, attracts the planet, and in virtue of that attraction the planet would move in a certain path which could be determined with comparative ease. But the actual problem is by no means so simple. The planet is acted on by other planets; its orbit is thus deflected slightly from the simple form it would otherwise have; and while the orbit preserves a general resemblance to the ellipse, it is in reality a path of the utmost complexity. But still the mathematician can follow the planet; he can point out with accuracy where the planet was at any ancient date; he can show where it will be at any future date. It is the infinitesimal calculus, the invention of Newton and Leibnitz, which enables this to be done. By this most subtle and exquisite contrivance we attack the problem in detail. It is comparatively easy to find out the direction in which the planet is moving at any instant, as well as its velocity. This will enable us to ascertain where it will be in the next moment of time. We are then in the same condition as before, and we can repeat the operation and carry on this process as long as we like, and thus discover where the planet will be at any future date. The success of the process consists in attacking the question in detail. Is there not in this a striking analogy to the great principle of Darwin? In each case great effects are produced by the constant addition of innumerable small tendencies, all in the same direction. As the infinitesimal calculus of Newton has led us to a wonderful knowledge of the physical laws which regulate the universe, so the infinitesimal calculus of Darwin has afforded the solution of the profound problem presented by organic life.
It must have been with a glimpse of prophetic insight that Cuvier exclaimed, 'Shall not natural history some day have its Newton?' At the very time these words were uttered the Newton of natural history had been born, and his immortal work has just been closed.
1 A Lecture delivered at the Midland Institute, Birmingham, on November 20, 1882.
2. Darwin, Descent of Man, p. 147.