Considerations relative to the Age of the Earth's Crust.
[Read before the Otago Institute, 9th December, 1924; received by Editor, 19th December, 1924; issued separately, 6th March, 1926.]
In considering such questions as this we may roughly divide our knowledge of relevant facts into two classes—namely, (1) things which are demonstrated, and (2) things which are proved by the balance of evidence. They are not really two classes, as the inferior value of the second is only a question of the strength of the evidence.
Under the first head we have—
(1.) Life began on this planet: if it also began elsewhere, that does not concern us. There could be no actual connection between the life that began here and the life we may guess began elsewhere. Of such life we have no proof, and if we could import living beings from elsewhere it would not alter the problem.
(2.) Life began at some date: that is to say, there was a time in the earth's history when it was too hot to allow the beginning of life, as we understand the term, to be possible.
(3.) Changes on the hot surface of the planet resulting in a cool surface and condensed waters made the advent of life possible. No form of life that we can conceive of, and consequently no form of life as we understand the expression, can exist on the sun or on the glowing stars, and it is improbable that it exists on Mercury or Saturn. Ascertainable conditions suggest this improbability.
(4.) It began in a sense spontaneously—that is to say, under some natural impulse governed by antecedent conditions affecting the matter which became a living thing. We cannot scientifically postulate any other mode of beginning. This fact may belong to the first or to the second class. We accept it as a fact because reason teaches us to regard it as proved by substantially cogent evidence negativing any other hypothesis.
(5.) Save that we must shut off the early stages of the earth's existence when its surface was unfit to support life, we cannot do more than conjecture when it began. We can only collect such evidence as is available denoting periods when life existed. We must now recognize a tendency to push this further and ever further back in the history of the planet as our actual knowledge increases.
Of the facts made out by strong if not cogent evidence we have—
(6.) Life began by a concourse of matter under chemical, physical, and perhaps electrical conditions favourable to certain aggregations of matter attaining a quality which we call life. Personally I do not see how this can be effectively disputed. It cannot be denied save by substituting something more probable for this hypothesis. The further we go back in the history of speculations as to the origin of life—or, rather, of living beings—
the more crude do we find these speculations. Milton describes the process from his point of view, veiled as to the origin but visible as to the result:—
The grassy clods now calv'd; now half appear'd
The tawny lion, pawing to get free
His hinder parts, then springs as broke from bonds,
And rampant shakes his brinded mane; the ounce,
The libbard, and the tiger, as the mole
Rising, the crumbled earth above them threw
In hillocks: the swift stag from underground
Bore up his branching head: scarce from his mould
Behemoth biggest born of earth upheav'd
His vastness: fleec'd the flocks and bleating rose,
As plants: …—Paradise Lost, Bk. 7. 463.
If seriously produced in our time, this description of the creation would provoke a smile; but I feel inclined to regard it with respect. It throws into poetic form the view of creation which for a century and a half after Milton wrote was the only known view.
(7.) It seems to me more reasonable to conclude that the aggregations of matter so conditioned as to be ready to receive life were very small rather than large. This conclusion I regard as of the utmost importance in connection with the question of geological time. We now know of the existence of living things so small that it is far under the mark to say, as was recently said of typhoid germs, that a thousand millions of living beings only aggregate to the size of a small pin's head. Why should we assume that they have degenerated to this size rather than that they were brought into being on this or on a much smaller scale? The presumption appears to me to be in favour of my proposition. Under this head we look to presumption until we can have demonstration.
If we venture without the support of any evidence to assume that life began in creatures or aggregations of greater size, how far are we to go? Are we to support the Miltonic creation? Where are we to make our assumed starting-point? I submit that in any case we cannot intelligently postulate anything larger than something still only perceptible with the aid of a microscope. Against a contrary assumption is the fact that certain diseases appear to be caused by poisons emitted by micro-organisms so small that our most vigilant observers have not yet seen them. Smallpox is one of these diseases. It begins and runs a course so like that of other fevers that, using our reasoning-powers, we conclude that it begins as other fevers begin, under the influence of a minute living organism capable of creating or acquiring and exuding a poison. Other familiar fevers, such as scarlet fever and measles, have a similar origin and run parallel courses. One of the most remarkable is trench fever, produced by organisms which are parasites in lice. This organism, like the others, has escaped visual observation. It has, however, been trapped and utilized to render patients immune from its own attacks. It is a reasonable conclusion from what is known of it that it is a highly specialized creature of ultra-microscopic dimensions.
These experimental observations were made during the late war, but more recent experiments have produced definite visual results. I need not go into the subject of ultra-microscopic observation and photography beyond mentioning that this latest triumph in scientific methods results in our obtaining visual cognizance of things that are too small to be seen under the most powerful microscope used in the ordinary manner. By this means it has been found that the microbe of foot-and-mouth disease, which has again appeared among cattle in Europe, belongs to the little-known group of “filter-passers,” organisms so minute as to be inseparable
by a Pasteur filter from a liquid containing them. This discovery is the work of two Dutch scientists, Paul Frosch and H. Ahmen, who pronounce the causative organism of this disease to be a yeast-like organism propagated by budding. Grown on a solid medium it gave rise to colonies from 7 to 8 microns in diameter. A micron is a millionth of a metre. The investigators report: “With high magnifications and the use of light of the shortest possible wave-length it was found possible to get photographs of individuals of the organism which proved to be bacteria with an estimated length of about one-tenth of a micron.” (English Mechanics, 11th July, 1924.)
It is not essential for my purpose to go further into this subject, as it cannot be determined to what extent the existing inhabitants of the earth are descended from creatures as small as this; but these investigations render it probable that others of the disease-producing germs are of this minute character, and that until reasons to the contrary are shown it is reasonable to assume that life began in creatures of very minute size.
The organisms which produce this and other diseases which I have mentioned are not likely to be of the original forms into which life was infused; they are all highly specialized poison-producers. In the case of foot-and-mouth disease the organism cultivated to the twenty-sixth generation has been found to retain its special virulence.
It seems to me not unreasonable to conclude that before life thus specialized as we know it began, countless generations in more primitive form had preceded the specialized forms, and that specialization involving increased size may have begun in forms more primitive than any we know.
At one time the habit prevailed of referring to amoeba as a primitive form; it is, however, a relatively large specialized form.
(8.) Is it not then a reasonable interpretation of the evidence to suggest that forms of life exist of the minute dimensions at which such forms, and perhaps all life, began?—that is, assuming that life did arise from organic matter. We need not assume that those very forms do not still from time to time come into existence. All that we can say is that we are wholly ignorant on the subject.
This course of reasoning may without violence even carry us down to those objects of which we now have actual cognizance, which we call ultra-microscopic objects.
It is not necessary, nor is it reasonable, to adopt any positive view on the actual question of size, but I think I have shown reasons for regarding it as probable that some of the germs of disease and other germs are of ultra-microscopic dimensions or of dimensions of a similar order when compared with visible living beings. I am content, however, to start with the assumption that life began where we reasonably conclude that it exists, because the qualities of beings that produce certain results are so like those of beings that we can see as to convince us that they are living beings. If that involves too great a draft on the imagination, then I am content to go down no further than to the size of beings which we can actually see—perhaps far below the size of the typhoid bacillus.
(9.) On these grounds I draw the conclusion that life began on a minute scale in that partly explored world of which we gain more knowledge every day. I speak of a conclusion, but I admit that the subject is so far speculative that any conclusion is liable to be upset. It would, however, be most difficult to explain away the case of the germ of foot-and-mouth disease, or to assert on reasonable grounds that it is a solitary case. In this connection it must be borne in mind that I am not writing on the beginnings
of life, but am attempting to discuss a different subject—namely, a line of inquiry as to the age of the earth's crust. Any one who found it impossible to believe that life originated as I have suggested, and held that the balance of evidence led to an entirely different conclusion, would not find that to discard this would affect the main theme of this paper. Darwin never found it necessary to discuss the subject of the origin of life. He dealt with the history of organized creatures. I am—I hope, with becoming modesty—discussing a parallel topic. It is therefore quite incidentally that I pause to point out that great chemists and biologists—I need only refer to such names as John Tyndall and Henry Chorlton Bastian—may be said to have advanced a long way apparently in the direction of showing how life began. It is, however, sufficient to say that their brilliant experimental efforts, while producing what must be regarded as organic from inorganic substances, and going far to break down the barrier between the inorganic and the organic, have not resulted in the production of living things. Speaking with the greatest respect of these and other great men of science, I find myself driven to say that while throwing down one barrier they have perforce left the other standing. The synthetic production of organic substances by Tyndall has been immensely advanced and widened in the hands of his disciples, but we still await the Frankenstein who can make them live and move and reproduce their kind. We do not know the origin of life; we can only speculate as to whether we ever shall know it. For the present, however, on mere arithmetical grounds based on the ordinary law of probability, and opposed by no more rational conclusion, I have provisionally formed this conclusion:—
(10.) When we attempt to speculate on the age of this planet as the home of living things we are forced to consider the line of reasoning with which I started. Any one may reject it, but he ought to do so either by showing some fallacy appearing ex facie or by producing evidence displacing it. That is the course of reasoning by which any scientific proposition other than a mere dogma must be destroyed or displaced. Incidentally, recent observations in the field of palaeontology may be referred to.
It has long been the habit of geologists to divide the stratified rocks of the earth into two categories. The oldest was called Azoic, when and because, so far as observation went, it was devoid of indications of past life. The second embraced all the rocks in which fossils and other evidence of life were found, from the earliest to those at present in course of formation.
Some geologists have attempted to give the relative thickness of the rocks in these two categories. More than one published scale shows the Azoic rocks as equal to one-half of the whole known series of stratified rocks.
The pre-Cambrian rocks of Scotland are 8,000 ft. to 10,000 ft. thick, resting on a further series of great thickness. A thick series of slates and phyllites lies below the oldest Palaeozoic rocks in central Europe, with coarse gneisses below. In America, about the great lakes, a vast succession of rocks of pre-Cambrian age has been classed as forming six successive layers of great thickness. These have received names, the lowest being the Laurentian. Some of them have, however, been found to contain traces of living organisms. Since 1910–11, the date of the eleventh edition of the Encyclopaedia Britannica, further explorations have yielded remarkable results. Some of these are summarized in the recently added Supplement of 1922. In this summary an attempt is made to apply a time-scale to the whole period covered by the growth of stratified rocks.
The term “Azoic” is not used, but, allowing sixty millions of “time units” for the building-up of all the periods, one-half is allotted to the pre-Cambrin ages. Here below the Laurentian is a vast assemblage of rocks awaiting further examination and exposition. When this stupendous series of old-world rocks is contemplated it is not surprising that geologists stubbornly stood out against the suggestions that limited the age of the earth's crust to a few tens of millions of years. In discussing the question of age the biologist necessarily goes hand-in-hand with the geologist.
It is when attempts are made to measure time by years that the greatest difference arises between the authorities. Here the physicist claims a voice, and in his present mood entirely reverses the contentions made a few decades back, when he relied on mathematical calculations which appeared to give but a relatively short life, both in the past and in the future, to the heat of the sun. Thus “taking the Lead as all produced by Uranium at the rate [elsewhere] above given we get an age of 925 million years. Some minerals from other Archaean rocks in Norway give a rather longer age … the upshot is that radio-active methods of research indicate a moderate multiple of 1,000 million years as the duration of the earth's crust as suitable for the habitation of living beings, and that no other considerations the side of pure physics or astronomy afford any definite presumption against this estimate.” (Lord Rayleigh, 1921.)
This method, it is said, carries back the appearance of Eohippus, the oldest form of horse, some thirty million years. Obviously this stretches the Tertiary ages in an enormous degree, making them cover seven or eight times as long a period as was not long since regarded as necessary. I have often wondered why the geologists and palaeontologists spoke of the Tertiary ages as extending over four million years. Why not forty million? I had to be silent in the presence of such unanimity—for unanimity there was. I am afraid that I am growing less modest as my years advance. Charles Dolittle Walcott, one of the most successful geological explorers of modern times, appears (1893) to speak of three million years for the age of mammals, while Joseph Barrell (1917) calls for fifty-five or sixty-five million years. The date (1893) given for Walcott's deduction was long prior to his remarkable discoveries. The modern tendency undoubtedly is to recognize immensely longer periods for the building-up of the rocks than were admitted a couple of decades back. Opinion on this comparatively new subject is, however, necessarily in a state of flux; we may not unreasonably hope for something more like demonstration as data accumulate and methods improve. Recent investigations by Walcott have revealed evidence of primitive life in the pre-Cambrian (Proterozoic) rocks of North America. In Montana, at a depth of nearly 10,000 ft. below the earliest Palaeozoic rocks (Cambrian), he found evidence of ancient reef deposits of calcareous algae which ranged upwards through 2,000 ft. of strata. The general barrenness of pre-Cambrian rocks is no doubt attributable to their extensive metamorphosis, presumably under the influence of heat-producing upheaval and other movements. This would tend to destroy everything but the hard casings of calcareous plants and of animals similarly protected. We need not, however, despair of the discovery of what may be termed the portraits of some of the even earlier denizens of the earth. Corals and sponges represent advanced structures—for Walcott has added to our knowledge of such life the discovery even of a monad or bacterium allied to Micrococcus living in pre-Cambrian times. Bastian had in 1910 referred to the fact that B. Renaut had found traces
of Micrococci and Bacilli “even as low as the upper Devonian strata.” This discovery has been immeasurably surpassed by those of Walcott. If any value can be attached to the presumptions I have ventured to put forward, even a Micrococcus may from its structure be regarded as a creature which has already advanced in point of size at least far beyond the most primitive forms of life.
These considerations encourage further investigation, which may carry the discovery of evidences of life not only down throughout the Laurentian age but into and perhaps through a period equally long which lies below it. It is impossible now to speak conclusively of an Azoic age so long as we are dealing with stratified rocks.
Incidentally it must be allowed that, apart from the suggestions of the physicists, we can have no actual data for fixing the age in point of time of the earliest rocks until we have means of determining the rate of the deposit of strata. This may, but need not necessarily, have been more rapid in earlier than in later times. There may have been, and probably was, a vastly greater rainfall, causing a more rapid destruction and redeposit of the surface soils. We may also have to reckon on tidal action on a far more active scale if the history of the rocks carries us back to a period when the moon was appreciably nearer to the earth than at present. Before this can be touched we must come to an understanding with the astronomer.
I may now briefly refer to certain forms of which we have definite knowledge, and concerning which we may fairly speculate as to their having in the course of ages grown up in successive generations from some of the smallest to some of the greatest living creatures.
The recent expedition to the Ross Dependency found it necessary, for economic purposes, to weigh their captures. The great blue whale was found to weigh 150 tons. If the assumption with which I have started has any foundation, this animal by a long process of evolution may have progressed from an ancestor a thousand million of which would build up a speck the size of a pin's head. This is not proved, but it is not disproved, and it lies within the limits of reasonable argument. There are, of course, difficulties. A dormouse weighing an ounce has presumably an analogous history. In its strcture it has an obvious affinity with its great congener. They are both mammals, and each has seven cervical vertebre in common with almost all mammals. In the course of its evolution it has been found to be to the advantage of the race to which it and its ancestors belonged to remain small, while it has been to the advantage of the whale branch to attain and retain an immense size. Such differences for protective and generally benefit-conferring purposes are familiar throughout animal life. Trees great and small afford a similar illustration. The difference between the kiwi, living in dense forests in New Zealand, and the moa, formerly frequenting open plains, is another familiar instance, the relation between these two birds being much closer than in the first-mentioned instance—so close, indeed, as to leave open no question as to their common origin.
As to the rate of growth in the course of evolution, of which we know so little, we are not confined to one group in our search for animals of immense size. Great developments are scattered through the ages immediately preceding our own. Our largest surviving land mammal is the elephant, and in no other order is there now a near approach to its size. Baluchitherium, the vertebrae and limb-bones of which were found
in the Upper Oligocene of Baluchistan, was probably much larger. In Pleistocene times the ground-sloths of South America rival the elephant and rhinoceros in bulk. The Megatherium was 18 ft. long, and its skeleton was of a most ponderous character. But perhaps the most remarkable of the mammals of the pampas is the great armadillo, Glyptodon claviceps, with the enormous weight of armour in separate plates over its body, head, and immense tail. It was 9 ft. long. The ponderous carapace indicates the prolonged operation of some cause or impulse, not solely connected with its feeding-habits, which had secured the preferential persistence (in survival of the retention) and progressive expansion of this monstrous development. Changes in the climate of this region probably produced a scarcity of food which led to the extinction of this great and varied fauna. The geological record as disclosed to us gives us at best but a hint in the shape of evidence. The pictures of past life, consisting not merely of skeletons, but of the undigested contents of the stomachs of the animals of the past, and even the fossilized eggs of the Dinosaurus recently (1924) discovered in northern China, are continually illuminating our means of study; but what lies buried beneath whole continents of land surface is only here and there disclosed by almost accidental breaches of that surface. What lies beneath vast stretches of ocean like that which separates southern Asia from Africa must remain for ever undisclosed so far as the human race as we know it is concerned. Thus far I have drawn my examples from the Tertiary period, with its specialized mammalian fauna.
We may, however, leave this and leap backward over the immense and unmeasured interval of time which separates the age of mammals from the age when the great reptiles were dominant. This interval is still unbridged. The prodigious development of the saurian fauna is the wonder of the geology of that great era. The largest surviving saurians, the crocodiles, are simply trivial when compared with their mighty predecessors—ancestors in any direct sense we can hardly call them.
The Brontosaurus excelsus of the upper Jurassic of Wyoming must have been fully 70 ft. long and 25 ft. high. Though its bones were lighter in structure than those of the great land mammals, it was built to walk on land, carrying its enormous weight on sufficiently stout legs. In the same region and of the same period is the Diplodocus carnegii, 80 ft. in length. Its skeleton shows that it must have had an immense muscular development to enable it to carry with comfort its enormous neck, projecting far beyond its body.
These two examples show size and weight, competing with that of our modern whales, carried by animals walking on land, even though living generally in the more sustaining medium of shallow waters. It is enough to mention these two examples of great vertebrates of the far past as showing that in that remote age great and wonderfully perfect development was achieved. *
To grasp the significance of these illustrations of the age in which the great saurians dominated the waters, the earth, and the air, we have to
[Footnote] * Since this paper was written further information has come to hand respecting the extensive discoveries made in recent times in the Tanganyika area of South Africa Amongst other important results these finds make it clear that the Gigatosaurus, with a humerus more than 7 ft. long, must have far surpassed in size any saurian or other form of animal theretofore revealed to the geologist. At least one writer claims for it a size double that of Diplodocus carnegii.
try our best to appreciate the immense lapse of time which has occurred since the Jurassic rocks were laid down to the enormous depth at which we find them.
I can only briefly—and, I must admit, in a most unsatisfactory way—refer to what I may term the arithmetic of the case. It is a subject which opens up a field for endless speculation and inquiry, but, so far as I know, it has not up to the present been very comprehensively attacked. How many generations are required under given conditions of temperature, environment, &c., to raise from an invisible microbe a Brontosaurus or a whale? The problem is a stupendous one, but it is one that cannot be passed over, and is one which will, I think, receive more attention in the future than it has received in the past. If a creature the size of a typhoid bacillus bred another which doubled its weight in the first generation, and again added its original weight in each succeeding generation, its progeny would by successful development reach the size of a pin's head in, say, for example, a thousand million generations. If this went on unswervingly, and there were ten generations in each year, the result would be achieved in something like a hundred million years. It is, of course, mere guesswork to talk of ten generations in a year; there might be a hundred. On the other hand, it is a bold step to suggest that life has run so smoothly that in each generation the descendant of any particular microbe added its original weight to its progeny. Probabilities are against this having occurred over the whole series of generations. If the ancestry of the whale could be traced to a creature of the earliest date of the age of mammals, or even to some creature approaching the mammalian type as small as those we are told are found as we approach the Eocene, and that ancestor were found to be no more than an ounce in weight, then it has in a given period, not stretching back into the great age of saurians, so increased its bulk that its descendants of to-day are more than five million times the weight of their ancestor. There is, moreover, here in the later stages no question of ten generations in a year. Some of the great mammals do not produce at the rate of one a year, after some years have been spent in reaching maturity. Such figures relate to a comparison in weight between two creatures of the relative sizes, say, of a mouse weighing an ounce and a whale weighing 150 tons. But without going into actual arithmetic it may be pointed out that the disparity in weight between a creature the size of a typhoid bacillus and one the size of a mouse is vastly greater; while between the size and weight of the invisible microbe of foot-and-mouth disease and the comparatively gross typhoid bacillus there may be a disparity approaching either of the above. It is difficult to grasp comparisons of sizes in dealing with such matters.
The growing importance of the subject of examining the minutest forms of life is, however, attested by the discoveries of Dr. F. d'Herelle, of the Pasteur Institute, which came under my notice while I was revising this paper. It is found that certain bacteria which propagate diseases—perhaps hitherto regarded as not curable—so small themselves that they are only revealed by means of a microscope of 3,000 magnifying-power, are preyed upon and destroyed by smaller parasitic organisms of the “filter-passer” type. So minute are these that they are described as “just as small in relation to ordinary bacteria as a flea is to a calf.” These have now been styled “bacteriophages.” They have already been utilized for the destruction of the bacteria of persistent dysentery, and their discovery, isolation, and culture give promise of a revolution in the treatment of various
diseases. (English Mechanics, 19th September, 1924.) What occurs to me in reading of such organisms is this: Are we to assume that they have retrograded from predecessors of larger size, or is it not more probable that they are rather in the vicinity of the size at which most lines of organized beings have received life ? Geological history, and analogies derived from it, give us the only clue we have at present to the answer. I will leave it to others, curious of arithmetical details, to calculate the relative weights or sizes of the various creatures involved in this discussion from a “filter-passer” to a Brontosaurus or a whale.
As we go down the geological ladder we find, as a general but not universal rule, that the creatures are getting smaller and smaller. It is true that examples of the smallest are always with us, while, as in the case of the giant saurians, the largest are sometimes left by the way; but the rule remains good until we find ourselves groping with Walcott among the immeasurably ancient rocks of Montana in the hope of here and there finding a legible stain or similar sign that life was once there. If our giant vertebrates are not descended from minute ancestors of those ages, we are left without means of even guessing where they came from. If they are so descended, have we not then to face the time-problem on some such lines as I have indicated ? The evidence points to the mammals having originated as small rat-sized creatures in the Jurassic and possibly, but doubtfully, in the Triassic period. That is to say, some of the saurians of the earlier period present some mammalian affinities; but to give this fact a definite value in this connection it would be necessary to show direct ancestral descent. It is considerations such as these which have led me to wonder whether our great geologists are right in assigning a period of only a few million years to the whole Tertiary age. I again most respectfully urge that it is probable that too much importance has been attached to evidence which appears to enforce this limitation.
I have presented certain problems of simple arithmetic. The least that can be said of them is that they deserve serious consideration. In doing so I have ignored evolution per saltum as I have ignored the Miltonic creation. I am quite prepared to admit that there may be an occasional sudden advance in size. We have, however, no knowledge of such a phenomenon. I am also quite alive to the observations of geologists who in certain periods of the earth's history find evidence of rapid expansion of groups of animals, as if new genera and species were appearing at an accelerated rate: Deprét has spoken of an “explosion” of forms in the ammonite genus Neumayria. Moreover, we can no more insist on a universal law of uniformity in the matter of speed of variation than on any other supposed fact of which we have no actual knowledge. What we do know as a general rule to be deduced from the evidence is that evolution as a whole has been slow and gradual, especially perhaps when dealing with large bony structures. When, therefore, we find in a single group what we take to be evidence of a more rapid rate of change we note it as something exceptional, accentuating, without contradicting, the general rule.
My modest efforts to illustrate what we require in the shape of vast measures of time are merely tentative, but again I claim that they call for discussion. My own impression is that when the matters to which I have addressed myself receive from competent investigators the consideration they deserve the result will emerge in the shape of conclusions as to the age of the crust of the earth of a totally different order from those heretofore admitted. My own opinion as to how such inquiries and discussions will probably eventuate is sufficiently disclosed in the above.