The Fitness of the Environment

ing dilution the conductance approaches a maximum value, nor, finally, the dilution law, was found to hold. Molecular weight determinations carried out at the same time by the boiling-point method gave normal values for non-electrolytes, and abnormally large values for electrolytes, whereas abnor- mally small values would be expected. This indicates that association has taken place, to a considerable extent, which in all probability takes place not only between molecules of dissolved substance, but also between these molecules and those of the solvent. Considering these circumstances, it is very fortunate for the advance of the sciences of chemistry and electro-chemistry that such complications are generally, although not always, absent in the case of aqueous solutions. It is due to this fact that it has been possible to deduce simple laws from a study of such solutions." — Le Blanc, "A Text- book of Electro-Chemistry." New York, 1907, pp. 142-143. • Full discussion of such subjects will be found in the "Dynamics of Living Matter," by Loeb, and in his contribu- tion to Oppenheimer's "Handbuch der Biochemie," as well as in HSber's "Physikalische Chemie der Zelle und der Gewebe."

and the sure and definite relations between such bodies provide, as it were, a secure foun- dation for the more complex organic structures. More obvious is the value of ions as sources of electricity. If the older electro-physiology of the third quarter of the nineteenth cen- tury has proved in some respects a sterile field, there can yet be no doubt that more subtly, and quite apart from the nervous im- pulse and the peculiar phenomena of electrical fishes, electrical phenomena are everywhere involved in the most intimate of the physio- logical processes.

Even without further discussion of a sub- ject that must soon lead into difiicult and highly technical considerations, I feel sure that the existence of another important fitness of water is patent. For ions are evidently a real contribution to the richness of the environment. They enhance the ■variety of chemical substances and of chemical reactions ; they constitute a group of singularly mobile chemical agents; they provide electricity; and, finally, aqueous solutions are by far the best source of ions.

the accompanying table Kq stands for the dielectric constant, Hy for the latent heat of vaporization, and K^ for the absolute conductivity for heat. It is to be observed that on the whole all three quantities decrease simultaneously. These properties are also related to the critical pressure, to the van der Waals constant a, and to the molecular volume at the boiling point. Methyl alcohol, CH3OH . . . Ethyl alcohol, CjHsGH . . . Formic acid, H • COOH . . . Acetic acid, GH3 • COOH .

Chloroform, CHCI3 .... Tetrachlormethane, CCI4 . . Stannic chloride, SnClj . . Such evidence clearly suggests that some of the manifold fitnesses of water proceed from a single cause or group of causes. For the present, however, these relationships are obscure, and in any case there seems to be no immediate hope of bringing them into connection with the science of biology. Of all common liquids, except mercury, water has the greatest surface tension.

This fact is of enormous' moment in biology, most obviously perhaps in its influence upon the soil. For surface tension and density determine the height to which a liquid will rise in a capillary system, and thus it comes about that the principal factor in bringing water within reach of plants is the exceptional surface tension of water. The nature of the case is clearly explained by Hilgard. "The liquid water lield in the pores of the soil, in the form of surface films representing the curved surface seen in capillary tubes, and therefore tending to cause the water to move upwards, as well as in all other directions, until uniformity of tension is established, is of vastly higher importance to plant growth than hygroscopic moisture. It not only serves normally as the vehicle of all plant food ab- sorbed during the growth of the usual crops, but also, as a rule, to sustain the enormous evaporation by which the plant maintains, during the heat of the day, a temperature sufficiently low to permit of the proper op- eration of the processes of assimilation and building of cell tissue."^ ■■]

The rise of water in capillary systems re- sembling soil, under the action of surface tension, may be as much as ten feet. In soil itself the highest rise under the usual circum- stances is unquestionably as much as four or five feet; but if the surface tension of water were like that of most liquids it could be, under similar conditions, but two or three feet. There seems to be little doubt that the rise of fluids in tall plants is also in large part due to the action of surface tension, and accordingly it must be much favored by the

Finally, surface tension is of great impor- tance, indeed in simple cases is the one effective agent, in the phenomenon called adsorption.^ On the basis of thermodynamical considera- tions first developed by Willard Gibbs, it is easy to show that whenever the dissolution of a substance changes the surface tension of a solvent, the distribution of the dis- solved substance will not be strictly homo- geneous. If the solution has a lower surface tension than the solvent, the surface of the solution will become more concentrated than the interior; or if the surface tension of the solution be greater than that of the solvent, the surface of the solution will become less concentrated than the interior. This result, quite insignificant in simple solutions, becomes a matter of much moment when, as in the case of suspensions of fine particles like ani- mal charcoal, in emulsions, jellies, or any other system of like disperse heterogeneity of physical constitution, there occurs very great increase of surface area. Then it is

' A familiar example of adsorption is the use of bone- black to decolorize sirup in the process of sugar refining. The colored matters are almost completely removed from solution, and cling to the surface of the charcoal. that adsorption becomes a factor of the great- est weight; for, other things being equal, the total force of surface tension in a system is proportional to the area of surface. Under these circumstances dissolved substances are no longer distributed with any approach to equality or regularity in the system, but they collect at the surface in very great quantities, and in the most irregular manner.

Now of all known physical structures there is none which rivals protoplasm in its fine complexity, and adsorption is therefore un- questionably a prominent agent in deciding its physico-chemical constitution. Mgreover, adsorption influences and complicates almost every process of chemical physiology, for no product of life is without its colloids, i.e. sub- stances which are finely divided and there- fore endowed with great surface areas. In truth colloids are probably quite essential to fine complexity, and so to every conceivable form of life.^

1 "Eines aber mochte ich behaupten, welches aucli immer die stoflSiche Zusammensetzung jener Lebewesen (living organisms in another world) sein mag : es mlissen KoUoide sein. . . . Welcher andere Zustand, ausser dem Kolloiden, konnte derart veranderliche, derart plastisehe Formen bilden und wSre doch im stande, diese Formen, wenn notig, miver- anderlieh zu wahren." — Bechhold, l.c., p. 194. of adsorption in biology is not to be briefly presented, but it may be found in almost endless profusion in such works as those of Freundlich and Bechhold.^

It must not be supposed that the phenomena of adsorption in biology are simple and exactly understood. What is certain is that they are universal, and that surface tension lies at the root of the matter. This is because all living things are colloidal, and I am in- clined to think that most physiologists will admit that life without colloids is probably unthinkable, even in a world very differently constituted from our own. Colloidal struc- tures are, in fact, the first and greatest factors in physical complexity of organization, and the principal force, unless it be in exceptional cases an electrical charge due to ions, which operates upon the colloidal structures is sur- face tension. This, then, is another striking fitness of water above all other things. *

Such are the facts which I have been able to discover regarding the fitness of water for the organism. The following properties ap- pear to be extraordinarily, often uniquely, * Freundlich, "Kapillarchemie." Leipzig, 1909. Bech- hold, "Die Kolloide in Biologie und Medizin." Dresden, suited to a mechanism which must be complex, durable, and dependent upon a constant metabolism : heat capacity, heat conductiv- ity, expansion on cooling near the freezing point, density of ice, heat of fusion, heat of vaporization, vapor tension, freezing point, solvent power, dielectric constant and ionizing power, and surface tension.^

In no case do the advantages which these properties confer seem to be trivial; com- monly they are of the greatest moment; and I cannot doubt, even after allowances have been made for the probability of occa- sional fallacies in the development of an argu- ment which, though simple, is beset with many pitfalls, that they are decisive. Water, of its very nature, as it occurs automatically in the process of cosmic evolution, is fit, with a fitness no less marvelous and varied than that fitness of the organism which has been won by the process of adaptation in the course of organic evolution.

* Contrasting this statement with that of Whewell in his Bridgewater Treatise, it is evident that while the progress of science has provided much novel information, and elim- inated many false views, the present situation differs from the earlier one only in the better definition of the issue, and in our modern freedom from metaphysical and theological associations. any other substance which, however slightly, can claim to rival water as the milieu^ of simple organisms, as the milieu interieur of all living things, or in any other of the count- less physiological functions which it performs either automatically or as a result of adapta- tion.

In truth Darwinian fitness is a perfectly reciprocal relationship. In the world of modern science a fit organism inhabits a fit environment. ^ Here and elsewhere the word " milieu " has been employed when it is desired to express oidy that part of the conception of environment which is involved in the literal meaning of the word, leaving to environment the added significance of that which provides food. TWO chemical individuals stand alone in importance for the great biological cycle upon the earth. The one is water, the other carbon dioxide. The one, for reasons which we have just reviewed, is the most familiar of all the varieties of matter. The other rarely is seen except by chance, and with- out scientific research never could have been known for what it is in value to living things. Yet these two simple substances are the com- mon source of every one of the complicated substances which are produced by living beings, and they are the common end prod- ucts of the wearing away of all the constitu- ents of protoplasm, and of the destruction of those materials which yield energy to the body.^

1 A man weighing 60-70 kilograms excretes daily the fol- lowing quantities of material : — Once perhaps the atmosphere of the earth consisted chiefly of water and carbon dioxide; but coohng has caused the condensation of most of the water, and geological processes, more recently aided by the action of vegetation with coal and peat formation, have removed nearly all of the carbon dioxide. The latter transformations have resulted in the substitu- tion of oxygen in the atmosphere. However, the interior of the earth continues to deliver through volcanoes large amounts of carbon dioxide, and thus the original source of atmos- pheric carbon dioxide persists as a diminish- ing supply.

To-day carbon dioxide makes up only a little more than 0.03 per cent by volume of the whole atmosphere, approximately 4.6 kilograms per square meter of the earth's surface, or about 2,300,000,000,000 metric Water is, tkerefore, three fourths, carbon dioxide one fifth, » of the total, and all other substances amount to but 2 to 3 per cent. In like manner the materials ingested by an ordinary green plant are proportioned, water making up more than nine tenths, and carbon dioxide amounting to fully five times the sum of all other substances combined.

Needless to say, a large part of the water which enters and leaves plants and animals has had no real share in their organization. It is merely the bearer of dissolved substances or the means, through evaporation, of lowering the tempera- ture of their bodies. tons for the whole earth. ^ In the ocean the amount of carbonic acid is about 0.1 gram per liter, or approximately 0.01 per cent by weight. Here, however, much the larger part is in chemical union with bases, chiefly in the form of bicarbonates.^

There can be no doubt that the physical properties of carbon dioxide are less important to the living organism than are those of water. But indeed the characteristics of water so largely determine the nature of the environ- * See below in the discussion of the alkalinity of the ocean. ment and the conditions within the organism alike, that this is necessarily the case. How- ever, the less conspicuous substance is not without its physical fitnesses, and we must now turn to them.

The most obvious of the properties of car- bonic acid is its all-pervasiveness. Originally formed in vast quantities by the cosmic pro- cess, and accumulated in the atmosphere, the store has been steadily replenished there by vulcanism. It is probable that of the enor- mous quantities now deposited as limestone in the earth's crust, a quantity sufficient to yield an atmospheric pressure greater ten- fold than the present atmospheric pressure, only a fraction was at any one time actually gaseous. For it happens that the presence of carbonic acid in the atmosphere insures the occurrence of greater, or at least nearly equal, quantities in the ocean and in all the natural waters of the earth. This is due to the solu- bility of carbon dioxide, to the magnitude of its absorption coefficient in water.

The absorption coefficient is the volume of gas absorbed by one liter of liquid when the gas pressure is one atmosphere. Accord- ing to the law of Henry, however, the absorp- tion of the gas is always proportional to its partial pressure in the atmosphere, and there- fore the absorption coefficient measures the ratio, after equilibrium has been attained, between the concentration of a substance in solution and in the g^aseous state, no matter what that concentration may be.

The absorption coefficient is not constant under all circumstances, and varies especially with the temperature. It will be seen from the tables that, differ- ing from most gases, carbon dioxide has an absorption coefficient nearly equal to one, and that for the ordinary temperature of the earth's waters, where they are in contact with car- bonic acid gas, it is very close indeed to 1.0, Hence, when water is in contact with air, and equiHbrium has been estabhshed, the amount of free carbonic acid in the water is almost exactly equal to the amount in the air. Unlike oxygen, hydrogen, and nitrogen, carbonic acid enters water freely; unlike sulphurous oxide and ammonia, it escapes freely from water. Thus the waters can never wash carbonic acid out of the air, nor the air keep it from the waters. It is the one substance which thus, in considerable quantities relative to its total amount, everywhere accompanies water.^ In earth, air, fire, and water alike these two sub- stances are always associated.

Accordingly, if water be the first primary constituent of the environment, carbonic acid is inevitably the second, — because of its solubility possessing an equal mobility with water, because of the reservoir of the at- mosphere never to be depleted by chemical * "Carbonic acid being more soluble than the other gases, is contained in rain water in proportions between 30 and 40 times greater than in the atmosphere." — Geikie, "Text- book of Geology." 4th ed., Vol. I, p. 449, 1903.

It must not be forgotten that carbonic acid in subterr?inean water, by which so much geological change is accomphshed, originates, not in the air, but from organic matter in the soil. action in the oceans, lakes, and streams. In truth, so close is the association between these two substances that it is scarcely correct logi- cally to separate them at all; together they make up the real environment and they never part company. Carbonic acid thus possesses the first great qualification of a food: its occurrence is universal and its mobility a maximum. This is due to the fact that its absorption coefficient is on the average ap- proximately one, the most favorable value.

Needless to say the absorption coefficient of carbonic acid is also of great importance in many physiological processes, chiefly per- haps in excretion. In the course of a day a man of average size produces, as a result of his active metabolism, nearly two pounds of carbon dioxide. All this must be rapidly re- moved from the body. It is difficult to im- agine by what elaborate chemical and physical devices the body could rid itself of such enor- mous quantities of material were it not for the fact that, in the blood, the acid can cir- culate partly free ^ and, in the lungs, by a pro- cess which under ordinary circumstances has all the appearances of a simple physical phe-

1 Of the total carbonic acid of the blood 6-10 per cent exists as the free acid, partly in the plasma, partly in the cor- puscles. nomenon,^ can escape into air which is charged with but Kttle of the gas.^ Were carbon dioxide not gaseous, its excretion would be the greatest of physiological tasks; were it not freely soluble, a host of the most universal existing physiological processes would be im- possible. The only other property of this substance which calls for consideration at this point is its acid nature and strength. Very few min- erals are freely soluble in pure water, and nearly all yield to the process of weathering far more readily because of the carbonic acid which all natural waters contain.^ The latter

1 The determination of the exact nature of the process by which carbonic acid is excreted across the lung membrane is one of the standing difficulties of physiology, but we need not here enter upon its consideration. ^ Expired air contains on the average more than 4 per cent by volume of carbonic acid. ^ "A few minerals (halite, for example) are readily soluble in water without chemical change, and without the aid of any intermediate element; hence the copious brine-springs of salt regions. In the great majority of cases, however, solu- tion is effected through the medium of carbonic acid or other reagent. Limestone is soluble to the extent of about 1 part in 1000 of water saturated with carbonic acid. The solution and removal of lime from the mortar of a bridge or vault,

constituent is probably a necessary adjuvant in the most far-reaching geological phenom- ena. Indeed, it is the united action of water and carbonic acid, aided in lesser degree by nitric acid, which has been formed in the atmos- phere by electrical action, and by acid prod- ucts of vegetation, which sets free the in- organic constituents of the earth's crust and turns them into the stream of metabolism. But apart from the solvent action of car- bonic acid, there is another group of phenomena which depend upon its acid character. These must now be explained. They are the neu- trality or faint alkalinity of the ocean, and of protoplasm.

According to the modern theory of solution, water itself, like the dissolved electrolytes, is dissociated into ions, though only to a very slight degree.^ The reaction is expressed as follows: — and the deposit of the material so removed ia stalactites and stalagmites, likewise the rapid effacement of marble epi- taphs in our chmrch yards, are instances of this solution. . . . Among the sulphates, gypsum is the most important example of solution. It is dissolved in the proportion of about 1 part in 400 parts of water. Even silica is abstracted from rocks by natural waters." — Geikib, "Geology," pp. 451-452. 1 For a discussion of this subject the textbook of Mellor, "Chemical Statics and Dynamics," p. 206, may be consulted.