Alfred Russel Wallace : Alfred Wallace : A. R. Wallace :
Russel Wallace : Alfred Russell Wallace (sic)

 
 
How Life Became Possible on the Earth
(S646: 1907)

 
Editor Charles H. Smith's Note: This article was commissioned for inclusion in the first volume of the Harmsworth History of the World, a popular readership-oriented eight-volume collection edited by Arthur Mee, J. A. Hammerton, and A. D. Innes. The original typesetting of Wallace's essay includes some apparent editorial interjections in the form of regularly spaced section headings; these have been included here in bold type at the approximate point at which they appeared in the text, near the left margins of each column. I have omitted a fold-out illustration of the "history of life" that appeared without pagination after page 96 of Wallace's text; Wallace does not refer to it nor is there any reason to think he created it. Original pagination of the article indicated within double brackets. To link directly to this page, connect with: http://people.wku.edu/charles.smith/wallace/S646.htm


    [[p. 91]] Early writers on the relation of man and animated nature to the material universe not only assumed that the latter existed for the former, but that both alike were the results of special acts of creation.

    Furthermore, they usually took it for granted that all things were created very much in the condition in which we now see them, and that any changes that have since taken place are but slight superficial modifications of a permanent and unchanging whole. Not only were the sun and moon and stars created as appanages of the earth, but the earth itself in all its details of sea and land, hills and valleys, mountains and precipices, swamps and deserts, was made and fashioned just as we now see it, and every feature of its surface was supposed to have some purpose in connection with man.

    These purposes we could, in some cases, understand, while in others they seemed wholly unintelligible, -The Old Ideas of Creation- and much ingenuity was bestowed by the natural theologian and others to explain more and more of the observed facts from this point of view. The same opinions prevailed in regard to the infinite variety of animals and plants, each individual species being supposed to have been an independent creation, and all to have some definite and preordained purpose in relation to mankind.

    These views, however absurd they seem to most people now, were almost universally held so recently as during the seventeenth and eighteenth centuries, and were thus coincident with one of the most brilliant epochs of our literature and our dawning science. It was only towards the beginning of the nineteenth century, when geology became widely studied and its results were fully appreciated, that the more rational conception of a very slow development of the earth's surface during countless ages began to be generally accepted.

    The grand nebular hypothesis of Laplace came to reinforce the views of the geologists, by showing how the earth itself may have originated as a gaseous or molten globe; and its slow process of cooling, with the reaction of the interior and exterior on each other, served to elucidate the facts of the heated interior, as shown by hot springs and volcanoes, as well as many of the phenomena presented by the distorted -Changing Conditions of the Earth- and metamorphosed strata which formed its crust. Hence it gradually came to be perceived that the condition of the earth, with all its endless variations of surface, of continents and oceans, of seas and islands, of vast plateaux and lofty mountain ranges and extensive low and plains, with their ravines and cataracts, their great lakes and stately rivers, was subject to perpetual change from that remote epoch when it seems to have been actually the case that "the earth was without form and void," and that owing to the greater density of the vapour-laden atmosphere, "darkness was upon the face of the deep."

    Another field of geological research forced us to the conclusion that the same continued process of change had affected the forms of life upon the earth. When carefully investigated, the crust was found to abound in the fossilised remains of animals and plants. Careful study of these showed that the oldest of all were of comparatively simple structure, and that the higher forms only appeared in more recent epochs; while the highest of all were probably very little older than man himself. -Changing Forms of Life- It is only during the last half century that the theory of Evolution has been elaborated and has become generally accepted as applicable to the whole of the vast cosmic process--from the development of the nebulæ into stars and suns and systems, with a corresponding development of planets from an early condition of intense heat, through a more or less lengthy period of cooling and contraction, to an ultimate [[p. 92]] state of refrigeration, the earlier and later stages being alike unsuited to the existence of life.

    More important still, the discovery of the theory of Natural Selection by Darwin--and at a later period by myself--has led to a satisfactory explanation of the successive appearance of higher and more complex forms of life, -Theory of Natural Selection- and also of that wonderfully minute and complex adaptation of every species to its conditions of existence and to its organic as well as its inorganic environment, which all other theories--even the most recent--have failed to grapple with.

    The logical completeness as well as the extreme simplicity of this explanation of organic evolution has led great numbers of thoughtful but ill-informed persons to reject it, because it seems to render unnecessary the existence of a primary intelligent cause; while another equally large but, as I think, equally ill-informed class--the so-called monists--use it to demonstrate the non-existence, or, at all events, the needlessness, of any such cause. Both alike err, because they fail to take cognisance of the fact that every form of evolution, and pre-eminently that of the organic world, is an explanation of a process of change, a law of development, not in any sense or by any possibility an explanation of fundamental laws, causes, or origins. It presupposes the existence not only of matter--itself a thing whose nature is becoming more and more mysterious and unthinkable with the advance of physical science--but of all the vast complex of laws and forces which act upon it--mechanical, physical, chemical, and electrical laws and forces--all more or less dependent on the still more mysterious, all-pervading ether. Thus, the universe in its purely physical and inorganic aspect is now seen to be such an overwhelmingly complex organism as to suggest to most minds -Wonderful Complexity of the Universe- some vast intelligent power pervading and sustaining it.

    Persons to whom this seems a logical necessity will not be much disturbed by the dilemma of the agnostics--that, however wonderful the material universe may be, a being who could bring it into existence must be more wonderful, and that they prefer to hold the lesser marvel to be self-existent rather than the greater. When, however, we pass from the inorganic to the organic world, governed by a new set of laws, and apparently by some regulating and controlling forces altogether distinct from those at work in inorganic nature; and when, further, we see that these organisms originated at some definite epoch when the earth had become adapted to sustain them, and thereafter developed into two great branches of non-sentient and sentient life, the latter gradually acquiring higher and higher senses and faculties till it culminated in man--a being whose higher intellectual and moral nature seems adapted for, even to call for, indefinite development--this logical necessity for some higher intelligence to which he himself owes his existence, and which alone rendered the origin of sentient life possible, will seem still more irresistible.

    The preceding remarks are intended to suggest that the theory of evolution, combined with the quite recent and very startling advances in physical science, so far from making the universe around us more intelligible as a self-sustaining and self-existent whole, has really rendered it less so, -Mind Behind the World- by showing that it is infinitely more complex than we had formerly supposed; and further, that matter itself, instead of being, as was once believed, a comparatively simple thing, eternal and indestructible, is in all its various forms subject to decay and disintegration. We now see that the only thing known to us that we can conceive as having unending existence is mind itself; and, just as Darwin's theory of Natural Selection has opened up to us an infinite field of study and admiration in the forms and colours and mutual relations of the various species of animals and plants, so does modern science open up to us new and unfathomable depths in the inner structure of matter and of the cosmos, and thus compels us more and more to recognise a mental rather than a mere physical substratum to account for its existence.

    There is, however, another set of relations which have been hitherto very little studied--those between the organic and the inorganic worlds in their broader aspects. These are now found to be very much more complex and more remarkable than is usually supposed, and they also have an important bearing upon the great problem of the origin and destiny of man. This is a subject [[p. 93]] which opens up a variety of considerations of extreme interest, showing that the exact adaptations of our earth--and presumably of any other planets--to enable it to sustain organic life, from its first appearance and through its long course of development, is as varied and complex and as much beyond the possibilities of chance coincidences as are any of the individual adaptations of animals and plants to their immediate environment. Most of these latter adaptations have been made known to us by Darwin and his followers, and they have excited the admiration and astonishment of all lovers of Nature. When the antecedent and grander relations of planet to life are studied with equal care, these also will, I believe, excite deeper admiration, still more profound astonishment, because any secondary laws that could have brought them about are less easy to discover, or even to imagine.

    Before we can form any adequate idea of the nature of a world which shall be able to support and develop organic life, we must consider what are the special conditions -Essential Conditions of Life- that alone render such life possible. We, of course, refer to the whole of the organic world, from the lowest to the highest, not to the few exceptional cases in which life may be possible under conditions that would be fatal to the higher as well as to most of the lower forms.

    The one striking speciality of the higher animals--and to a less degree of the higher plants--is that of continuous, all-pervading motion, every portion of their substance being in a state of flux: each particle itself moving, growing, living and dying, and being replaced by other particles of the same nature and fulfilling the same functions. To keep up this growth, and to enable every part of the structure to be continually renewed, food is required. This is taken into the stomach of animals in the solid or liquid form, is then decomposed and recomposed, that which is useless or superfluous being thrown off by the intestines, while what is needed for growth is transformed into blood and by a wonderfully intricate system of branching tubes is carried to every part of the body, furnishing nourishment and repair alike to bone and muscle, to all the internal organs and all the outward integuments, and to that marvellously complex nervous system which also permeates every part of the body and is essential to the higher manifestations of life--to the exertion of force, voluntary motion, and, apparently, to thought itself. Add to this the constant influx of air, which at once purifies the blood and supplies animal heat, and is so important -The Miracle of Human Life- that its cessation for a few minutes is usually fatal, and we have a machine so complex in its structure and mode of action that the most elaborate of human machines is but as a grain of sand to a world in comparison.

    Now the very possibility of such a material organism as this depends upon a highly complex form of matter termed protoplasm, which is at once extremely plastic and of extreme instability, and is yet capable of secreting or building up its atoms into such solid and apparently durable forms as bone, horn, and hair, besides the various liquids and semi-solids which build up the organism. This fundamental organic substance consists of only four chemical elements--nitrogen, hydrogen, oxygen and carbon, and almost all animal and vegetable structures and products have the same elemental constitution, though with such widely different characteristics. Four other elements--sulphur, lime, silicon, and phosphorus--also occur in small quantities in organic tissues, to supply special needs; but these are not essential to all forms of life, and are only taken up and utilised by the living protoplasm when required. Protoplasm is undoubtedly the basis of physical life, yet it only exists in, and is produced by, living organisms. The moment such an organism dies, disorganisation and decay set in, and the whole mass becomes gradually changed into more stable compounds, or into its constituent elements. It appears, therefore, that some agency--usually termed "vital force"--must be at work, -Basis of Physical Life- first to produce this wonderful compound, then to form it into "cells"--the physiological units of all organisms--and afterwards to direct the energies supplied by heat and light so as to build up the excessively complex structures, with all their wonderful powers and potentialities, which we term animals and plants. All this seems to imply not "a force" only, but very many forces, all of which must [[p. 94]] have some kind of mind in or behind them, to direct these forces to such infinitely varied yet perfectly defined ends.

    Consider for a moment one of the simplest of these cases. Let us take the minute seed of one of the great tropical fig-trees, and another seed of a strawberry, or of garden cress. Both will be about the same size and shape, -A Marvel of Every Day- and the most acute microscopist would not find any difference in the internal structure that could intelligibly account for the different results when these little grains of protoplasm are exposed to identical conditions. For, even if planted near each other, and exposed to the same amount of heat and moisture, to the very same atmosphere, and the same kind of water, as well as identically the same soil, yet invariably the one will grow into a large tree, the other into a small herb, and in the course of time, still with no change whatever of the physical conditions to which both are exposed, each will produce its peculiar foliage, and flowers, and fruit, very different in all their characters from those of the other. Were this result not so common as to seem to us "natural," we should call it a miracle; and it is really and essentially as inexplicable as many things which are termed miracles only because they are unfamiliar and inexplicable.

    Now, this wonderful substance, the physical base of all life--and as it is the only base that exists, or has ever existed, on the earth, we may fairly assume that no other is possible--can only maintain itself and perform its functions under certain very definite conditions, which conditions are now maintained on our earth's surface, and must have been maintained throughout the long geological periods during which life has been slowly developing. What these conditions are we will now proceed to show.

    -The First Essential for Life- The first essential for organic life is a certain very limited range of temperature. We are so accustomed to consider the change of temperature from winter to summer, from day to night, and that which occurs when we pass from the tropics to the Polar regions as being very great, that we do not realise what a small proportion such changes bear to the whole range of temperature that exists in the known universe. The absolute zero of temperature is calculated to be minus 461° F., while the heat of the sun has been determined to be over 10,000° F., and many of the stars are known to be much hotter than the sun. The actual range of temperature is therefore enormous; but any development of organic life is possible only within the very narrow limits of the freezing and boiling points of water, since within those temperatures only is the existence of liquid water possible. But a much less range than this is really required, because albumen, one of the commonest forms of protoplasm, is coagulated or solidified at a temperature of about 160° F. Now, if, as is generally believed, the earth has been once a liquid or even a gaseous mass and has since cooled to its present temperature on the surface, and the sun is undergoing a similar process of cooling, we are able to understand that the very limited range of temperature within which life development is possible implies an equally limited period of time as compared with that occupied by the whole process of solar and planetary development.

    -We Live by the Heat of the Sun- It must be understood, however, that the present temperature of the earth's surface is due entirely to sun-heat, and that if that were withdrawn or greatly diminished the whole surface of the globe would be permanently far below the freezing point and all the oceans be frozen for a considerable depth; so that all organic life would become extinct. Under such conditions no renewed development of life would be possible; and it is therefore quite certain that the sun has actually maintained the uniform moderate temperature required, and must continue to maintain it for whatever future period man is destined to continue his existence upon the earth.

    But it is not only a certain amount of heat that is required, but also a sufficient quantity of light; and this implies a further restriction of conditions, because light is due to vibrations of a limited range of wave-length, and without these particular rays plants cannot take the carbon from the carbonic acid in the atmosphere, and by its means build up the wonderful series of carbon compounds, including protoplasm, which are essential for the life of animals. What is commonly termed dark heat, therefore, would not be sufficient for the development of any but the [[p. 95]] lowest forms of life, even though it produced the necessary temperature during a sufficient period of time.

    All organisms, from the lowest to the highest, whether plant or animal, consist very largely of water, and its constant presence either in the liquid or gaseous form is essential for organic life. On our earth oceans and seas occupy the greater part of the surface, while their average depth is so great that the quantity of water is sufficient to cover the whole of the globe free from inequalities two miles deep. It is this enormous amount of water that supplies the air with ample moisture, such as renders the life of the tropics so luxuriant. Yet even now the inequality of water-supply is such that large areas in all parts of the earth are what we term deserts, only supporting a very few forms of life that have become specially adapted to them, and certainly unfitted for the continuous development of life from lower to higher forms.

    Water is also of immense importance as an equaliser of temperature, the currents of the ocean -Water and the Atmosphere- conveying the warmth of the tropics to ameliorate the severity of temperate and Polar regions, while the amount of water-vapour in the atmosphere acts as a retainer of heat during the night, without which it is probable that the surface of the earth would freeze every night even in the tropics. When we consider that water consists of two gases--oxygen and hydrogen--in definite proportions, and that without their presence in these proportions and in the necessary quantity the development of organic life would have been impossible, we find that we have here a remarkable and very complex set of conditions which must be fulfilled in any planet to enable it to develop life.

    But this is not all. The atmosphere is so intimately associated with water in its life-relations, and is itself so absolutely essential to the existence from moment to moment of the higher animals, that the two require to be duly proportioned to each other and to the globe of which they form a part.

    In the first place the atmosphere must be of a sufficient density, this being needed in order that it may be an adequate storer up of solar heat, and also in order that it may be able to supply sufficient oxygen, water-vapour, and carbonic-acid gas for the requirements of both vegetable and animal life. We have a striking example of the use of air as a storer-up and distributor of heat and moisture in the very different character of our south-west and north-east winds. The effect of the density of the air is equally well shown when we ascend lofty mountains where we find -How Water Protects Earth by Night- perpetual snow and ice, due simply to the fact that the air is not dense enough to retain the heat of the sun--which is actually greater than at low levels--so that at night the temperature regularly falls below the freezing point. On the other hand a very much denser atmosphere would absorb so much water vapour as probably to shut out the light of the sun, and thus have a prejudicial effect on vegetable life.

    Again, there is good reason to believe that the proportions of the various gases in the atmosphere are, within certain narrow limits, such as are most favourable not only for the life that actually exists, but for any life that could be developed from the elements that constitute the universe. Oxygen has properties which seem absolutely essential to organic life; but nitrogen, though only serving to dilute the oxygen so far as the higher animals are directly concerned, is yet indirectly essential for them, since it is in vegetables a constituent of that protoplasm which is the very substance of their bodies.

    Now, plants obtain their nitrogen mainly from the minute proportion of ammonia that exists in the atmosphere, and this ammonia is formed by the union of the nitrogen of the air with the hydrogen of the water-vapour under the influence of electric discharges--that is, of thunderstorms. It is evident, then, that the required amount of this essential compound will depend upon a due adjustment of the quantities of nitrogen and aqueous vapour always present; while the electric discharges -Use of Thunderstorms- seem to be due to the friction of various strata of air with each other and with the earth's surface, due to the winds and storms; and winds are due to highly complex causes, involving the rate of the earth's rotation, the rise and fall of the tide, the density of the atmosphere, the quantity of its aqueous vapour, and the amount of solar heat which it receives. Unless all these very diverse factors existed in their due proportion, some of the results [[p. 96]] might be highly prejudicial if not quite inimical to the development of life. To these various adaptations of our gaseous envelope we must add one other. Carbonic acid gas in the atmosphere is absolutely essential to vegetable life, while it is directly antagonistic to that of the higher animals. Its quantity must, therefore, be -The Wonder of the Atmosphere- strictly proportionate to the needs of both; and that beneficial proportion must have been preserved throughout the whole period of the existence of the higher air-breathing animals.

    These various considerations show us that our atmosphere, consisting as it does mainly of two common gases mixed together, and therefore seeming to most people one of the simplest things possible, is really a wonderfully complex arrangement which is adapted to serve the purposes of living organisms in a great variety of ways. But this by no means exhausts the subject of its adaptation to support and develop organic life, because its very existence on the earth in a suitable quantity and composed of the essential elements can be shown to depend on other and deeper relations which will now be pointed out.

    The older writers on the subject of the habitability of the planets took no account whatever of the importance of size, distance from the sun, period of rotation, and obliquity of the ecliptic as determining the possibility of organic life, but simply assumed that, because the earth possessed an abundant life-development, all the other planets must also possess it. But we know that the above-mentioned factors are of very high importance, as we will proceed briefly to point out.

    It is now believed that the amount of atmosphere possessed by a planet is due mainly, perhaps entirely, to the planet's mass, and its consequent gravitative power. Spectrum-analysis has shown that vast masses of gaseous matter exist -Earth's Envelope of Gas- in the universe, and it is probable that, in a state of extreme tenuity, these are very widely diffused. Just as meteoric dust is constantly attracted to the earth, and periodically in larger quantities, so are gases, and supposing the aggregations of free gaseous matter to have been distributed with some approach to uniformity, then, as planets grew in size, they would also tend to secure a larger amount of the diffused gases, thus forming deeper atmospheres. The observed facts agree with this view. The largest planets, Jupiter and Saturn, have such a depth of atmosphere as permanently to obscure any solid interior they may possess. The only planet closely approaching the earth in size and density--Venus--has an atmosphere which appears to be loftier than ours, but it may be composed of different gases. Mars, which has only one-ninth the mass of the earth, has a lofty but very tenuous atmosphere, and probably no water, the Polar snows being due probably to the freezing of some dense gas. The climate and physical condition of Mars is, however, still a subject of much controversy, which I hope to discuss in a separate work dealing with the arguments of Professor Lowell [see page 105]. In that volume the reader will find, fully set forth my reasons, on scientific grounds, against the supposed habitability of Mars.

    But, besides attracting cosmic masses of gaseous matter to form its atmosphere, there is another equally important function of the mass of a planet--its selective power -The Earth Selects and Uses Gas- on the kind of gases it can permanently retain in a free state. The molecules of gases are in a condition of rapid motion in all directions, which explains the elastic force they exhibit. The speed of this motion has been determined for all the chief gases, and also the gravitative force necessary to prevent them from continually escaping into space from the upper limit of the atmosphere. Thus the moon, which has a mass only one-eightieth that of the earth, can retain no free gas whatever on its surface. Mars can retain only the very heavy gases, but neither hydrogen nor water-vapour. The earth, however, has force enough to retain all the gases except hydrogen, which is just beyond its limit; and this may explain why it is that there is no free hydrogen in the atmosphere, although this gas is continually produced in small quantities by submarine volcanoes, is emitted sometimes from fissures in volcanic regions, and is a product of decaying vegetation. Once united with oxygen to form water, it becomes amenable to gravity in the form of invisible aqueous vapour, and is thenceforth a permanent possession for us in its most valuable form.

    The very accurate adjustments that render our earth suitable for the production [[p. 97]] and long-continued development of organic life, culminating in man, may be well shown by another consideration. If our earth had been 9,600 miles instead of 8,000 miles in diameter--a very small increase in view of the immense range of planetary magnitudes from Mercury to Jupiter--with a slight proportionate increase in density, due to its greater force of gravitative compression, its mass would have been about double what it is now. This would probably have led to its having attracted and retained double the amount of gases, in which case the water produced would have been double what it is--perhaps even more, because hydrogen gas would not then escape into space as it does now. But the surface of the globe would have been only one-half greater than at present; so that, unless the ocean cavities were twice as deep as they actually are, the whole surface of the earth--except, perhaps, a few tops of submarine volcanoes--would have been covered several miles deep in water, and all terrestrial life would have been impossible.

    From the various considerations here set forth -The Deep Atmosphere of Venus- it appears clear to me that no other planet of the solar system makes any approach to the conditions essential for the development of a rich and varied organic life such as adorns our earth. One only--Venus--has a sufficient bulk and density to give it the needful atmosphere; but as it receives about twice as much solar heat as does the earth, it is probable that its very deep atmosphere may be mainly due to the fact that a large proportion of its water is held in a state of vapour, its seas and oceans being proportionately reduced in extent. Judging from what happens on the earth, this would probably lead to an excessive area of deserts, and thus be inimical to life. But this planet appears to possess one feature which renders it fundamentally unsuitable for organic life.

    Several modern observers have found that the older astronomers were all in error in giving Venus a rotation-period almost exactly the same as ours, an error due to the indefinite and variable markings of its surface. They have now deduced a period about equal to that of its revolution round the sun--a rate which has been confirmed by spectrum-analysis, and further confirmed by the fact that this planet has no measurable polar compression. As during transits of Venus over the sun's disc the conditions for the accurate measurement of the compression, if any exist, are the best possible, and as none has been found, this alone affords a demonstration that the rate of rotation must be very slow, because the laws of motion necessitate a definite amount -Why there is no Life on Venus- of equatorial protuberance corresponding to that rate. Half the surface has, therefore, perpetual day and the other half perpetual night, leading to violent contrasts of heat and cold for the two hemispheres with, in all probability, correspondingly violent winds, rains, and electrical disturbances--conditions so entirely opposed to the uniformity of temperatures and stability of meteorological phenomena during long geological epochs which are essential for the full development of organic life, that such development is perhaps less probable on this planet than on any other.

    I think I have now shown not only that no other planet in the solar system makes any approach to the possession of the varied and complex adaptations which are essential for a full development of organic life, but also that on the Earth itself the conditions are so numerous and so nicely balanced that very moderate deviations in excess or defect of what actually exists in the case of any one of them--and of others not referred to here--might have rendered it equally unsuitable, so that either no organic life at all, or only a very low type of life, could have been developed or supported.

    If, then, the more superficial indications of design in the relations of animals to their environment, and of man to the universe, have been shown by modern science to have required no special interference of a higher power to bring them about, but that they have been due to natural laws acting in accordance with and in subordination to the deeper laws and forces that determine the very constitution of matter and the unknown power -There is Purpose in our World- and principle we term "life,"--yet, on the other hand, we find that a more careful study of the outer universe, or cosmos, reveals a new set of adaptations not less wonderful or more easily explicable by chance coincidence than those presented by the organic world.

    Even the very brief sketch of the subject here given suggests the idea of purpose in a world so precisely and uniquely [[p. 98]] adapted to develop organic life, and to support that life during the countless ages required for the completed evolution of man. But that suggestion becomes a logical induction when the whole of the available evidence is set forth, as I have attempted to set it forth in my work on "Man's Place in the Universe." I have there shown not only that the cumulative evidence for the earth being the only supporter of a fully-developed organic life within the solar system is irresistible, but that there is some direct, and much more indirect, evidence that this uniqueness extends to the whole stellar universe; and it is certain that no particle of direct evidence for the existence of organic life elsewhere has been, or is likely to be, adduced.

    I have also shown (in an appendix to the second edition of my book) that the purely biological argument for the uniqueness of the development of man--as the culminating point of one line of descent throughout the diverging ramifications of the animal kingdom--is overwhelmingly strong; hence the logical conclusion from the whole of the evidence is that man is the one supreme product of the whole material universe.

    My object in the present essay has been limited to showing that, besides and beyond the special adaptations of the various kinds of animals and plants to their special environments, there exist in the earth as a planet, in its various physical and cosmical relations, a whole series of adaptations of a very remarkable character which, so far as we can judge, are essential to its function as a life-producing world. The study of these adaptations, therefore, may be considered to be appropriate here, as constituting a preliminary chapter in the natural history of the Earth and of Mankind.


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