The Fitness of the Environment

Quite as important to man as this great power of meteorological regulation is the corresponding physiological activity, evap- oration of water from the skin and lungs. In an animal like man, whose metabolism is very intense, heat is a most prominent ex- cretory product, which has constantly to be eliminated in great amounts, and to this end only three important means are available:- conduction, radiation, and the evaporation of water. The relative usefulness of these three methods varies with the temperature of the environment. At a low temperature there is little evaporation of water, but at body temperature or above there can be no loss of heat at all by conduction and radiation, and the whole burden is therefore thrown upon

evaporation. The manner in which evapora- tion becomes important in temperature reg- ulation is well illustrated by the following cal- culation from a chart of Rubner's.^ The experiments upon which the chart is based were made upon the dog, an animal which lacks man's apparatus of sweat glands. The values of the table are only approximate. In plants evaporation is even more impor- tant than in animals. Evidently such adap- tation of the physiological processes to the conditions of the environment is enormously favored by the high latent heat of evapora- tion.'^

1 Lusk, "The Science of Nutrition," p. 99. ^ A full discussion of this subject will be found in the work of Lusk (see above), Chap. Ill, especially pp. 98-100. There is still another beneficial result of this property: the great variation in the vapor tension of water which accompanies variation in temperature. Vapor tension measures the amount of vapor which is present in the atmosphere when it is in contact with a liquid and after it has become saturated with the liquid's vapor. Now, according to a well- known law, the rate of increase of vapor ten- sion, or in other words the amount of vapor which the air can hold, is greater the greater the latent heat of vaporization.'^ Hence, degree by degree there is more variation in the vapor tension of water than there could be if the latent heat were lower. Such great variability in the quantity of water which the air can hold is in meteorology the most important characteristic of aqueous vapor.^ The relationship between vapor tension and temperature (centigrade) is shown in the accompanying table.

' Near the freezing point an increase of 10° in temperature doubles the amount of water which the air can hold. The increase is proportional to the latent heat of vaporization according to the formulae 2.3025 log El = I8:W/T^^\ ^ PC 1.99 V ToTi / where W stands for the latent heat of vaporization, p for vapor tension, and T for temperature. These variations are what make possible both the evaporation of water and its precipitation as rain and as dew in the meteorological cycle. And therefore the high latent heat of vapori- zation of water is in still another manner a most favorable circumstance in its effect upon the organisms.

To sum up, this property appears to possess a threefold importance. First, it operates powerfully to, equalize and to moderate the temperature of the earth; secondly, it makes possible very effective regulation of the tem- perature of the living organism; and thirdly, it favors the meteorological cycle. All of these effects are true maxima, for no other substance can in this respect compare with water.^ ' This conclusion may be contrasted with that of Whewell (Bridgewater Treatise, p. 142). He includes the expansion of water by heat, the expansion of water by cold below 40?. the ^^ansion of water in freezing, the latent heats of melting and

The heat conduction of water is also a maxi- mum among ordinary liquids, and, though very low compared with good conductors like metals, must favor the equalization of tempera- ture within the living cells whose structure hin- ders the establishment of convection currents. A final thermal property of water remains to be considered; namely, its anomalous ex- pansion when cooled at temperatures neaA- the freezing point. The facts are illustratecj by the accompanying table.

evaporation, and the rate of evaporation of water. It will be seen that eighty years ago it was already possible to make out a strong case for the fitness of water ; but it should not be forgotten that at that time ideas were in some respects still very vague, and comparative data few. This unique property of water is the most familiar instance of striking natural fitness of the environment, although its importance has perhaps been overestimated.^ If, how- ever, water, like all other common substances,

1 It scarcely merits tke curious .rhapsody of Prout, for instance : "The above anomalous properties of the expan- sion of water and its consequences have always struck us as presenting the most remarkable instances of design in the whole order of nature — an instance of something done ex- pressly, and almost (could we indeed conceive such a thing of the Deity), at second thought, to accomplish a particular object." — Prout, Bridgewater Treatise, "Chemical Meteor- ology and the Function of Digestion." London, 1834, pp.

steadily contracted on cooling, so that its point of maximum density fell at tlie freezing point, it is impossible to say how great would be the disadvantage for living organisms. Certain it is that life upon the earth would be thereby very greatly restricted. For this prop- erty, together with the by no means unique phe- nomenon of expansion upon solidification,^ is very largely responsible for the permanence in liquid state of many bodies of water in cold climates. In salt water the anomalous contraction disappears, and the lack of paleo- crystic ice is due to the density of ice and to the great mass of the ocean and the movement of its waters.^

There is an old experiment of Rumford's which well illustrates what conditions must have been had the contraction of water been normal and ice denser than water.^ He found that in a vessel filled with water, which contains ice confined at the bottom, it is possible to heat and even boil the superficial portion of the water without melting the ice. And so it would be with lakes, streams, and oceans were it not for the anomaly and the ^ The density of ice at the melting point is 0.91674. ^ A full discussion of this subject will be found in S. Giinther's "Handbuch der Geophysik." ' See WheweU's Bridgewater Treatise.

buoyancy of ice. The coldest water would contiipLually sink to the bottom and there freeze. The ice, once formed, could not be melted, because the warmer water would stay at the surface. Year after year the ice would increase in winter and persist through the summer, until eventually all or much of the body of water, according to the locality, would be turned to ice. As it is, the tempera- ture of the bottom of a body of fresh water cannot be below the point of maximum den- sity ; on cooling further the water rises ; and ice forms only on the surface. In this way the liquid water below is effectually pro- tected from further cooling, and the body of water persists. In the spring the first warm weather melts the ice, and at the earliest possible moment all ice vanishes.

Such are the important thermal properties of water, and in briefest outline their unique fitness for the living mechanism. No other known substance could be substituted for watery as the material out of which oceans, lakes, and rivers are formed, and as the sub- stance which passes through the meteorolog- ical cycl'e, without radical sacrifice of some of the most vital features of existing condi- tions. Ammonia in these respects is the only substance now known which approaches the

^fitness of water. But not only is it almost inconceivable that ammonia should ever jaccur in sufficiently vast quantities upon a planet's surface, but it is evident as well thalt am- monia wholly lacks the qualification of anoma- lous expansion, /and also in some of the most important of the other thermal properties falls far short of water ; while in latent heat of melting and in specific heat its advantage over water is inconsiderable. ^ It is obvious that upon a body like the earth the state of the oceans and the meteor- ological phenomena are of the utmost impor- tance to all living things. Unless these be favorable, human experience and reflection alike agree that life could not widely exist. It seems, therefore, almost safe to say, on the basis of its thermal properties alone, that water is the one fit substance for its place in the process of universal evolution, when we regard that process biocentrically.

Although the thermal properties of water make up the classical subject-matter for dis- cussions of the fitness of the natural en- vironment, other no less important physical properties exist. Such especially are those characteristics of liquid water which in no small measure determine the nature of the resulting physico-chemical systems when other substances, whether soluble or insoluble, crys- talline or colloidal, are brought into contact with it ; I mean the solvent power, the dielec- tric constant, together with the related ioniz- ing power, and the surface tension.

As a solvent there is literally nothing to compare with water. In truth its qualifi- cations are on this point so unique and ob- vious that nobody seems to have taken the trouble to gather together the evidence, and, accordingly, beyond the bare assertion, a brief ^ statement of the facts is not easy.^ In the first place the solubility in water of acids, bases, and salts, the most familiar classes of inorganic substances, is almost universal. * Nearly the whole science of chemistry has been built around water and aqueous solutions. A reference to any text- book wiU at once reveal the truth of this statement. At first sight such a condition appears to be a matter of chance, but, as one becomes more familiar with the true character of the science, realization of a rational justification for the historical fact steadily grows.

Relatively few of these bodies are highly insoluble; very many are exceedingly soluble in water. Apart from their electrolytic dis- sociation and hydrolysis, which will be later discussed, the chemical changes wrought upon such dissolved substances in solution are commonly very unimportant;^ For chemical inerthess, depending upon great stability, is a most significant characteristic of water, and undoubtedly a highly advantageous one- as well. On the whole the best evidence for the efficiency of water as a solvent of inorganic sub- stances is to be found in the data of geology. Of all geological agents water appears to have been by far the most active within the periods of which investigation is made pos- sible by the geological record.^ Rainfall, the movement of surface streams and of water beneath the ground, and wave action, all contribute to the work of disintegration, sedimentation, etc., partly by dissolution of soluble material, partly by mechanical action. But mechanical action is itself much in- creased by the loosening which earlier dis- solution has caused. In this manner the great solvent power of water throughout its meteor- ological cycle largely contributes, to the mobil-

ization of many materials which could not otherwise be brought to the organisms which need them. It has been calculated by Murray^ that the total yearly run off of all the rivers of the earth is about 6500 cubic miles, carrying nearly 5,000,000,000 tons of dissolved mineral matter and prodigious quantities of sediment. The average composition of such water has been estimated as follows : — It is, of course, almost exclusively to these constant accessions that the ocean owes its salinity, which in the course of time has reached well-nigh inconceivable magnitude. The common salt alone in the oceans of all

the earth amounts to not less than 35,000,000,- 000,000,000 tons. Quite as significant of the solvent power of water is the variety of elements whose presence in sea water can be demonstrated, thus proving that the total store of them is in any case enormous. They include hydrogen, oxygen, nitrogen, carbon, chlorine, sodium, magnesium, sulphur, phos- phorus, which are easily demonstrated; further, arsenic, caesium, gold, lithium, ru- bidium, barium, lead, boron, fluorine, iron, iodine, bromine, potassium, cobalt, copper, manganese, nickel, silver, silicon, zinc, alumin- ium, calcium, and strontium.^

Equally striking is the evidence in regard to the first stages of this geological process. Under the action of water, aided, to be sure, in many cases by dissolved carbonic acid, every species of rock suffers slow destruction. All substances yield in situ to the solvent work of water, and the dissolved parts may* all be found in the great final reservoir, the ocean. It has been proved that nearly every one of the substances which are thus set in motion upon the face of the earth are placed under contribution by life, for bio- chemical analysis reveals them as constitu- ents of living organisms, absorbed either

on their way down from the mountain tops to the ocean or by the marine flora and fauna. Not less valuable to the community of living things than the dissolution of the rocks, is the disintegration and transport of solid material, largely dependent thereon, which among its many results includes the prelim- inary steps of soil formation. By these familiar and enduring geological means chemi- cal substances are mobilized in the greatest variety of forms and conditions, and thus rendered available for the living organism. It is clearly evident from the chemist's long experience with solvents that no other fluid could permanently carry on this process with such acceptable regularity and eflSciency. For no other chemically inert solvent can compare with water in the number of things which it can dissolve, nor in the amounts of them which it can hold in solution; and of course any chemically active solvent must sooner or later exhaust itself by its chemical action, when the cycle must cease. Here, then, is a fitness of water which is open to no doubt.

Let us turn for further proof to the organ- ism itself, taking blood serum as a source of information. The composition of this sub- Most of these substances are in solution, and unquestionably a host of others are pres- ent with them, in small and varying amounts. Among these may be mentioned iodine, bromine, iron, sulphates, urea, ammonia, ben- zoic acid, amino-acids, etc. But indeed all substances found in urine (see below) also occur in blood. It cannot be doubted that^ if the vehicle of the blood were other than water, the dissolved substances would be greatly restricted in variety and in quantity, nor that such restriction must needs be ac- companied by a corresponding restriction of the life processes.

^ A full discussion of the following data may be found in such works as those of Hanunarsten -and Abderhalden on physiological chemistry. The composition of the urine provides another excellent illustration of the utility of the solvent power of water. In the course of its complex chemical processes a higher organism produces a host of end products which must be removed, and also finds itself accidentally in possession of a great variety of other useless substances which require excretion. The solvent power of water is one of the great factors in facilitating this task. Human urine has been reported to contain in solution the following stibstances: urea, carbamic acid, creatinine, creatine, uric acid, xanthine, guanine, hypoxanthine, adenine, paraxanthine, heteroxanthine, epi- sarkine, epiguanine, oxalic acid, allantoine, hippuric acid, phenaceturic acid, benzoic acid, phenolsulphuric acid, skatoxylsulphuric acid, paraoxyphenylacetic acid, homogentisic acid, urobiline, urochrome, uroerythrine, glucose, levulose, lactose, numerous compounds of glycuronic acid, glycine, alanine, leucine, tyro- sine, and other amino-acids, various enzymes, putrescine, cadavarine, and countless other organic substances, chlorides, bromides, and iodides, phosphates and sulphates, potassium, sodium, ammonia, calcium, magnesium, iron, carbonic acid, nitrogen, argon, etc.

not be attained with another solvent. It is no exaggeration to say that except atmos- pheric oxygen and carbonic acid, nearly all the food of living organisms is water borne, and all material in its passage into the body, through the body, and out of the body nearly always employs the same vehicle. Cer- tainly no other form of transport would be so efficient and so economical. If, therefore, aqueous solutions are, ap- parently of necessity, the very basis of the life processes, the state of substances when in this condition, and also when in contact with water, is of vital importance. Here two prop- erties of water, the dielectric constant and the surface tension, exert a cardinal influence.

Among the phenomena of solution those which are related to electrolytic dissociation, • as suggested by the hypothesis of Arrhenius, have deservedly received a great deal of at- tention since the secure establishment of the new science of physical chemistry in the eighties of the last century. In the course of time the belief that in aqueous solution the molecules of all acids, bases, and salts are more or less split into particles which bear electrical charges has been universally ac- cepted. These so-called ions are the source of nearly all the electrical phenomena of solu- tion, whether in batteries, in the manifesta- tions of animal electricity, or in simple con- duction through an aqueous solution. But the more familiar electrochemical processes are by no means the only results of ioniza- tion. An infinite number of chemical inter- actions between dissociated bodies follow inevitably. These changes are not, to be sure, decisive and irreversible, but balanced actions, which, however, vastly increase the variety of substances that exist in water.

Let us consider, for example, a system which has been made by dissolving in water the simple salts sodium chloride, NaCl, potas- sium bromide, KBr, and lithium iodide, Lil. According to the ionization hypothesis, more than half of the molecules of every one of these salts will at once dissociate into ions as follows: — These reactions are balanced, and it is confidently believed that the ions are con- stantly recombining to form molecules and the molecules constantly dissociating once more to form ions. At the same time, noth- ing hinders the union of sodium ions with bromine ions, or of any other pair of positive and negative ions. Accordingly, the solu- tion at once contains not only the three origi- nal salts and the six different varieties of ions, but also the following new molecules: — sodium bromide, NaBr, sodium iodide, Nal, potassium chloride, KCl, potassium iodide, KI, lithium chloride, LiCl, and lithium bromide, LiBr. All nine varieties of molecules and all six of ions are concerned in a complicated system of chemical reactions which are now well understood, the state of equilibrium depending upon known conditions. For in- stance, if the solution be a moderately dilute one and the original substances be present in chemically equivalent quantities, about 90 per cent of the material will be in the ionic state, each variety of ions making up about 15 per cent of the total, and about 10 per cent will be in the form of molecules, each variety constituting about 1.1 per cent.

There can be no doubt that ionization plays a great part in determining the char- acteristics of solutions of acids, bases, and salts, and in bringing about the reactions which Such, then, is the process to which are due most of the electrical phenomena and many of the chemical phenomena of solutions, and it is certain that the extent and variety of ionization in water far surpass what is possi- ble in any other solvent. One reason for this is most simple. The ionizing substances are so very much more often soluble in water than in any other solvent, and when soluble are in general so much more highly soluble, that the opportunity for ionization in water is quite unparalleled. Further, ionization in solution unquestionably depends upon^^the- dielectric constant of the solvent{ in accord- ance with the principle first stated by Nernst that the greater the dielectric capacity of the solvent, the greater is the degree of electro- lytic dissociation of substances dissolved in it, when the conditions are otherwise the same.^

1 "The following consideration will make this principle clearer : The positively and negatively charged ions would unite to form electrically neutral molecules because of the electrostatic attraction which exists between them if it were not for the action of another and opposing force, the nature of which is as yet unlcnown. The equilibrium between these two forces gives rise to the equilibrium between the ions and the undissociated molecules, or determines the degree of dis- sociation. When the dielectric constant is increased, the

This is the case because the tendency of the electrically charged ions to reunite and form electrically neutral molecules must be less the greater the dielectric constant of the sol- vent. Now the dielectric constant of water <> ^^s nearly the highest at present known, and therefore ionization in water is on that account also more extensive than in almost any other ^ Finally, for reasons that are not yet under- stood, the process of ionization in other sol- vents than water is a much more complex afifair, and there can be no doubt that such complexity limits the phenomena which are dependent upon ionization.^

electrostatic attraction between the ions is alone weakened, and hence the degree of dissociation is increased." — Lb Blanc, "A Textbook of Electro-Chemistry." New York, 1907, p. 147. ^ "It would be natural to expect that the conceptions which have been found serviceable in the case of solutions in water could be applied directly to solutions in other solvents, keeping in mind that, according to the individual nature of any given solvent, the degree of dissociation, the migration velocity of the ions, and consequently the conductivity of a solution of a given concentration would be different. It is a noteworthy fact, however, that the behavior of non-aque- ous is much more complicated than that of aqueous solutions. This is shown especially by the investigation of the conduc- tivity of solutionsof various substances in liquid sulfur dioxide made by Walden and Centnerszwer. Neither the law of the independent migration of the ions, nor the law that by increas-

Physiologically, as researches of the last twenty years clearly prove, the action of ions is of fundamental significance. The brilliant investigations of J. Loeb, and the long series of studies by various other physiologists of the influence of electrolytes upon colloids form perhaps the most telling evidence for this belief.^ At all events there is no ques- tion that the simple equilibria between acids and bases and salts are of extreme importance in physiological processes. They lie at the very basis of the structure of all protoplasm,