Elements of Physical Biology

its composition at different altitudes, up to 140 km., is given in table 12 and illustrated graphically in figure 41, derived from Humphrey’s work. A complete discussion of the réle played by the atmosphere in the round of terrestrial life would amount to nothing less than a treatise on meteorology, such as forms no part of the present project. What important factors weather and climate are in the business of providing the sustenance of life is a matter of common knowledge, and a very particular concern of the farmer. Yet we must here be satisfied with little more than a passing reference to meteorology, noting only a few elementary facts which bear directly upon the subject in hand, the circulation of the chemical elements in nature.

Losses from the Atmosphere. A first question that suggests itself in this discussion of the economy of nature is this: Since the atmosphere is “‘open‘at the top,”’ so to speak, is there not a loss, a constant leakage of gas out into space? The answer to this question must be sought in terms of the molecular constitution of the gases of our atmosphere. A cubic centimeter of air contains (at 0°C.) about 3.15 X 10'° molecules. These are in continuous agitation, somewhat after the manner of a swarm of gnats, except that they flit about with speeds comparable with that of a rifle bullet (about 500 meters per second) rather than with the leisurely flight of an insect. At a temperature of 0°C. a molecule of nitrogen has, on an average, a velocity of 492 meters per second. A molecule of hydrogen, under the same conditions, would have an average velocity of 1839 meters per second. It must be understood that these figures represent, in each case, a mean about which the velocities of individual molecules cluster, so that a certain proportion of them will fall below and others will exceed the figures stated. At the earth’s surface the average distance travelled between two successive collisions is about ;>p9,000 cm. But in the upper ranges of the atmosphere conditions are very different. If we follow the estimates and computations of J. H. Jeans, we find that at an altitude of 3200 km. the atmospheric pressure is reduced to about 1/10" of its value at sea level; but even at this low pressure there are still about 300,000 hydrogen molecules per cubic centimetre. (At this altitude all other gases except hydrogen are practically absent). The mean free path between collisions is now 10,000 km. or about 14 times the earth’s radius. In such circumstances collisions between molecules are rare, and for the most part the molecules move freely through space in parabolic or elliptical orbits, and become virtually diminutive satellites of the earth.

Since the day of Jules Verne’s story From the Earth to the Moon it has been a matter of popular knowledge that a body projected from the earth with a velocity exceeding 7 miles per second will go off in a hyperbolic orbit, never to return. ‘This applies to the molecules of a gas. Any of them that may be travelling outward with such a speed in the region where collisions are so rare as to be negligible, will leave the earth for good and will thus be lost to our atmosphere. The rate of leakage from the atmosphere thus depends on the number of molecules per unit of time that acquire the limiting (outward) velocity of 7 miles per second. This number, in turn, depends on the temperature in the region under consideration, a point regarding which our information is very uncertain. But an exact knowledge of this temperature is not needed to compute a major limit, a maximum figure which the rate of escape certainly cannot exceed. It

is thus found that under present conditions? the earth holds her atmosphere so effectively that there cannot be any appreciable leak even in many millions of years. Cosmic losses from the atmosphere then, are, for all practical purposes, wholly negligible. Certain other subtractions from and accessions to the atmosphere we shall have occasion to note as we consider the circulation of the several elements. As a matter of fact the composition of the atmosphere in the region in which living organism have their habitation is very nearly uniform’ and constant, except

*It may be noted in passing that in the earth’s past history conditions may have been different. If at any time the temperature of the upper atmosphere was about 750°C., then there must have been a very distinct loss of hydrogen by leakage into space. The moon, and certain of the planets having a lesser gravitational pull or a higher temperature (Mercury), have probably lost in this way any atmosphere that they may have had. For a detailed discussion of this and other points in connection with the escape of gases from the atmosphere the reader may be referred to J. H. Jeans’s Dynamical Theory of Gases, 1921, Chapter XV. See also E. A. Milne, Trans. Cambr. Phil. Soc., vol. 22, 1923, p. 483; J. E. Jones, ibid., p. 535.

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as regards its moisture content, and we may accept for the composition of the atmosphere at the earth’s surface, moisture excluded, the figures shown in the first two columns of table 13. The third column shows the total amounts of the several constituents of the entire atmosphere, according to W. J. Humphreys (Monthly Weather Review, vol. 49, June, 1921, p. 341). Cosmic Accessions to the Atmosphere. Meteorites falling upon the earth from space bring with them certain quantities of entangled or occluded gases. While this contribution to the atmosphere is at the present time, presumably, of negligible dimensions (see also page 195, Cosmic Accessions to the Lithosphere), yet in the course of the

long procession of ages past this source may not have been wholly insignificant. Data on this question are at best very uncertain, and a mere passing reference must suffice. The Hydrosphere. In comparison with the ocean all other aggregations of water upon the earth are insignificant in amount. The bald statement of the total volume of the ocean—302 million cubic miles—conveys but little to the mind. More impressive it is to recall that the average depth of the sea is 24 miles, and that, even if these waters were spread over the whole earth, leaving no continents, the average depths would still be 14 miles.

The average composition of the ocean is shown in table 14. Here again, the figure for the volume of total dissolved solids, 4.8 million cubic miles, is made more readily comprehensible by a graphic illustration. The salts of the ocean, made into one solid block, would cover the entire United States and Alaska to a depth of 11% miles; or according to J. Joly, they would encrust the whole earth to a depth of 112 feet.5 The Aquatic Atmosphere. Aquatic species perform their respiration in contact with an atmosphere of gases held in solution in the water that surrounds them. This atmosphere is very different both in concentration and also in composition from that in which we live, as is apparent from table 15.

A comparison of the several columns in table 15 is an object lesson on the adaptability of living organisms to varied conditions. The atmosphere in which fish and other marine animals live in comfort would not only drown us with its principal constituent, water, but, even if this were removed, the residual gases would suffocate us for lack of oxygen; and if the deficiency in this gas were made up by the addition of the amount required to bring the percentage up to that to which we are accustomed, we would still be choked by the excessively high percentage of carbon dioxide.®

The Lithosphere. Immense as the ocean appears to us, with its average depth of 23 miles, yet it constitutes less than rs'00 of the total mass of the earth, whose bulk is thus concentrated chiefly in lithosphere. Of the deeper layers of this lithosphere we have but scant and indirect knowledge. Earthquakes give evidence of some change in constitution about half way down to the center of the globe. Conditions and occurrences at such depth as this would seem, in the present state of our knowledge, to have little bearing upon the life at the surface. Other indirect evidence regarding the earth’s interior is derived as follows: The volume of the globe being known from triangulation, and its weight from direct determination with a balance’ or in other ways, the mean density of the earth is found to be

5. W. Clarke, loc. cit., p. 24; Sci. Trans. Roy. Soc. Dublin, 1899, vol. 7, eRe : 6 It must be admitted, however, that much of this CO: in sea water is partially neutralized by alkali. 7 A concise survey of the principal determinations of the mass of the earth will be found in J. H. Poynting’s little book (in the Cambridge Manuals series) The Earth (1913) It may add interest to the bald figures to note here in passing that the density of the earth’s crust (2.7) is not very widely different from that of the moon (3.46). This fact has a certain significance in connection with Sir Charles Darwin’s theory of the origin of the moon, according to which our satellite originally formed part of the earth and was thrown off by aspecies of tidal disruption. The bulk of the moon, then, would be formed of material derived from the outer layers of the earth.

Average composition of terrestrial matter* LITHOSPHERE HYDROSPHERE fase. (93 PERCENT) | (7 PERCENT) | ,owospHERE 5.5. But the mean density of the rocks outcropping at the surface® is only 2.7. Whatever may be the character of the earth’s interior, it is thus evident that it is composed of denser material than we find at the surface. In point of fact, comparison with meteorites, and other evidence, make it appear likely that the earth’s interior is virtually a metallic regulus (chiefly iron), encrusted with a slag not unlike that which separates out and floats on the molten mass of metal in a blast furnace. Such, essentially, is the raw material of our landscape, such our habitation, such the ground on which we tread and from which we draw the substance of our body. For the fertile soil in the plain is but the weathered variant of the granite strength of the hills, and we ourselves but a strangely metamorphosed portion of the world-stuff, the slag coating of a metal core.

Cosmic Accessions to the Lithosphere. The earth receives a constant shower of meteorites from interstellar space, at an annual rate of some 20,000 tons.? This seems a large amount. As a matter of fact, spread over the surface of the globe (197 million square miles), the accumulated meteoric material of a thousand million years would make a layer only 1 inch thick, if its density were that of water, ora correspondingly thinner layer of denser material.

Composition of the Earth’s Crust. Table 16 adapted from F. W. Clarke’s Data of Geochemistry and a more recent publication by Clarke and Washington, shows the average composition of the known terrestrial matter. The first column, in particular, gives the figures for the solid crust. This table exhibits a number of facts and relations of interest. Perhaps the first significant circumstance that strikes the eye, in glancing at the table, is the very unequal deal with which nature has distributed matter among the ninety odd! known elements. One-half the lithosphere, and one-quarter of the atmosphere are made up of the element oxygen.

The eight most abundant elements of the earth’s crust (oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium)—the only ones whose amounts are over 1 per cent—constitute together over 98 per cent of the earth’s crust. These, with hydrogen, titanium, carbon and chlorine— twelve in all—make up 99.5 per cent; thus leaving only one-half of one per Fig. 42. Comparison or Composirion or Harrn’s Crust AND Human Bopy cent for all the other elements, among them some quite indispensible for our existing civilization.

Relation to Composition of the Organism. Of the elements specifically significant for the living organism, only one, oxygen, is present in great abundance. Carbon, hydrogen and nitrogen, the principal “organic elements,’’ are among the less abundant constituents of the globe. On the whole it may be said the living organisms are composed of comparatively rare elements. We are, indeed, earthborn, but yet not altogether common clay.!? Thisis well brought out

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in the chart figure 42, which shows, side by side, the average composition of the known terrestrial matter, and, in comparison, the approximate composition of the human body. This latter is not exactly a representative sample of the totality of living matter (see tables 17 12 Indeed, taken literally the expression “common clay,” as applied to man, is an extreme case of poetic license; for aluminum and silicon—the chief constituents of clay, and taking second and third place in rank of abundance among the components of the earth’s crust, are both present only in

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Taking the mother earth as a whole, and the organism as a whole, it certainly cannot be said that there is much evidence of ‘‘inheritance Composition of the salts in sea water and in blood serum in per cent* PRTLITEIVIS I eG oo totes s Aare g Riis ewe arsearee e Na 30.59 | 39 PAPAS ERI ete aneret ym iacia oh corere cts or set se Mg 3.79 0.4 SP IPETTYE MAI Oe Sate oes BV pw ole ak a2 tee Ca 1.20 1 ARETE ee yee es ide Doe aes ae ae K WA Prot RUE aT ITs Bete ice tons ies Oe atte paw one bees, otis ae Cl BOA 45 SNe ALe WON sue ls qacasc eens ae ea 6a SO, 7.66

(arpDOnAle 1OBeece ve. ovawsr ses cee 25s os CO; 0.21 12 ECT Tt: ¢ Ay Gel Pe ei ore aanreaee Br 0.19 of parental characters” in their respective compositions. Still, resemblance is not wholly lacking. This becomes evident if we compare, not the entire organism with the whole of the earth’s crust, but the blood of a mammalian, for example, with the water of the ocean. This comparison is made in table 19 and figure 43. The likeness thus seen, imperfect as it is, can hardly be ascribed to accident. The fact is,

13 Silica furnishes, however, the skeletal support in a variety of living forms (radiolaria, sponges, plants). Cf. G. Bunge, Physiological and Pathological Chemistry, 1902, p. 23-24. Fora detailed discussion of the distribution of the chemical elements in organic nature see Vernadsky, Revue Gén. Sci. The reader interested in this phase of the subject should not fail to acquaint himself with this article, which came to the writer’s attention too late to be given more than this passing note here.

as pointed out by Palitzsch,' that aquatic species are in such intimate contact, both at their body surface, and more particularly in their gills, with the surrounding water, that the latter might almost be considered continuous with their bloody fluids, so that sea water may justly be “placed in the same category as the other physiological fluids.” “Frederiq! has shown that the amount of sodium chloride in the blood of crustacea varies, and all but corresponds, with the density of the water in which the creature has been kept.’’ More highly organized aquatic species have made themselves in greater degree independent of the salinity of their environment;" and finally, it would appear, when the marine ancestors of terrestrial vertebrates emerged from the sea and adventured life on dry land, they packed, as it were, a portion of their saline environment in their baggage, and took it along with them on their excursion as an essential part of their milieu intérieur. And to this day, according to this view, we ourselves carry about with us in our arteries and veins, if not a portion of the actual ocean, at least a roughly approximate replica of its brine. For, as L. J. Henderson!’ remarks: ‘‘Not only do the body fluids of the lower forms of marine life correspond with sea water in their composition, but there are at least strong indications that the fluids of the highest animals are really descended from sea water.’’ Some such indications may be seen in the reflections (conceived from a slightly different point of view) of G.

Bunge,'* “I am convinced that the remarkably high percentage of salt in vertebrate animals, as well as the desire to take salt with our food, can be satisfactorily explained only by the theory of evolution.”” In support of this consideration Bunge points out that in the weathering of rocks by the action of rain-water charged with carbonic acid, the sodium is dissolved and carried off as carbonate, while the potassium largely remains behind in combination with silica. The sodium carbonate being washed to

the sea. undergoes double decomposition with the alkali earth chlorides, these latter being deposited as carbonates (limestone and dolomite) while the sodium remains in solution as salt. Thus sea water is rich in sodium chloride and poor in potassium, while on dry land the balance is essentially reversed.'® Plants and invertebrate animals, Bunge points out, contain little sodium, unless they live in a highly saline habitat, in or near the sea, or on salt steppes. Yet the land vertebrates are all remarkably rich (comparatively) in salt, in spite of the scanty supply around them.

Is not the large amount of sodium chloride found in the present inhabitants of dry land another proof of the genealogical connection which we are forced to accept from morphological facts? There is no doubt that each of us in his individual development has come through a stage in which he still possessed the chorda dorsalis and the branchial arches of his sea-dwelling ancestors. Why may not the high average of salt in our tissues be also inherited from them?

Support for the supposition thus suggested is seen by Bunge in the fact that the younger a vertebrate is in its individual development, the more salt does it contain. Furthermore, cartilage contains the highest percentage of sodium of all the tissues of our body, and is also the tissue of greatest antiquity. The human skeleton is originally composed of cartilage, which is replaced, for the most part, by bone as the individual matures. These are facts which lead most readily to the interpretation that the vertebrates living on dry land originally came from the sea, and are still continuing to adapt themselves to their present surroundings, where they can get but little salt. We prolong this process of acclimation by taking advantage of the salt strata which have been left on the land by our primeval element, the salt flood.