The Fitness of the Environment

The stability of this substance justifies a belief that even far longer chains of carbon atoms can exist, and, in fact, there is no known limit to the possibility of stringing carbon atoms together. No other element is believed to share both of these characteristics, and there are various reasons to suppose that the resulting pecul- iarity of the system of organic compounds is really unique. Turning to the periodic classification of the elements, it will be seen that carbon is a member of the first series. Several of the elements of this series, unlike all the other elements except hydrogen, pos- sess very definite individual properties, which

mark them off sharply from other substances. Thus carbon bears relatively little resem- blance to its neighbors silicon or titanium, nitrogen to phosphorus or vanadium, oxy- gen to sulphur or chromium; while hydro- gen, of course, has a place quite apart in the classification, and as an element appears to be correspondingly unique. It is, therefore, in the highest degree prob- able that compounds made from elements of such positive chemical characteristics and very unusual properties will be unlike com- pounds formed from other elementary sub- stances. In this manner the periodic classi- fication confirms our confidence in the results of many decades of experience, which lead us to believe that other elements are exceedingly unlikely readily to form compounds com- parable in number, variety, and complexity with those of organic chemistry as we know it.

For more evidence we may turn to certain further data of organic chemistry. I refer principally to the character of the organic radicals composed exclusively of carbon and hydrogen. In making a rational classifica- tion of the carbon compounds it has been found convenient to commence with that series of hydrocarbons, called paraffines, with which the present discussion was begun. From them other series may be derived by making a substitution in the molecule. Thus the substitution of a hydroxyl radical for a single hydrogen atom leads from the paraf- fine hydrocarbons, C„H2„+2, to the alcohols C„H2„+iOH; the substitution of a carboxyl radical — COOH, for the methyl group — CH3, leads from the parajQfine hydrocarbons C„H2„+iCH3 to the acids C„H2„+i-C00H.

Moreover, the classes of compounds thus defined chemically fulfill the logical require- ments of a class. They are collections of well-characterized and very similar individual things which diflfer greatly, and in well-marked manner, from all other things. In other words, growing complexity of the molecule, when it consists only in increase in complexity of the simple radical comprised of carbon and hydrogen, of the formula C„'H.2n+u has very little effect upon the properties of the molecule. Thus the compound methane.

CH4, very closely resembles normal butane, CH3 • CH2 • CH2 • CH3 ; and again propionic acid, CH3 CH2 - COOH, and heptylic acid, CH3 • CH2 • CH2 • CH2 • CH2 • CH2 • COOH, are very much alike. Quite difiFerent is the case when any other radical accumulates in the molecule. For instance, propyl alcohol, CH3-CH2*CH2-OH, which closely resembles ordinary alcohol, CH3 -0112011, is very different in its behavior from glycerine, CH2OH CHOH • CH2OH, and similarly, acetic acid, CHs* COOH, di&ers materially in properties from oxalic acid COOH -COOH. Even more marked are the differences when a radical accumulates upon a single carbon atom. In successive stages of oxidation ethane, CH3 • CH3, yields alcohol, CHa- CH2OH, aldehyde, CUs' CH(0H)2, which by a secondary transformation goes over into the more stable form CHs-CHO, and acetic acid, CH3-C(OH)3, which similarly becomes CH3"C00H. These changes correspond to the conversion of ethane, that is monomethyl methane, into dimethyl methane, trimethyl and tetramethyl methane : —

In the latter ease the introduction of methyl groups in place of hydrogen produces no appre- ciable change in the general characteristics of the substances ; in the former, the succes- sive introduction of hydroxyl groups forms substances belonging to three diflferent classes of compounds, — alcohols, aldehydes, and acids, which have nothing in common. In short, variation in the number and arrangement of such groups as occur in the paraffine hydro- carbon.

is without manifest effect upon the more important properties of the molecule, but variation in the number and arrangement of any other groups produces complete change in its characteristic properties. It may perhaps be urged that this argu- ment is fallacious, in that the increase of the relative amount of hydroxyl in the above cases is larger than the relative change in radicals of the types — CH3, =CH2, =CH, and = C. But, in the first place, it is evi- dent that the latter four radicals are actually different, and a 'priori there is no reason to suppose that they should not greatly differ in their effect upon the properties of a mole- cule, for instance, to render dissimilar the compounds normal pentane CH3-CH2*CH2' CH2*CH3 and tetramethyl methane,

which is not the case. In the second place, the change from methane, CH4, to ethane, CH3 • CH3, is a larger proportional change in the molecule than the change from alcohol, CH3CH2OH, to glycol, CH2OHCH2OH, or aldehyde, CH3 • CHO, both of which produce far greater changes in the properties. In fact, the union of carbon with hydrogen in organic compounds is a unique and peculiar chemical relationship, upon which the proper- ties of the carbon compounds, their number, variety, and complexity largely depend. It seems to make no important difference whether

a carbon atom is attached to four hydrogen atoms, or to one carbon and three hydrogens, or to two carbons and two hydrogens, or to three carbons and one hydrogen, or to four carbon atoms ; in all such cases the_£Sect of the radical upon the general characteristics of the molecule va ries j v ery littl e. There are a great number of phenomena which might be employed further to illustrate the nature of the case, but two will suffice. The acidity of acetic acid, CHa'COOH, is only slightly and slowly changed by the accumulation of hydrocarbon radicals; thus the compounds propionic acid, CH3-CH2' COOH, and butyric acid, CHg-CHaCHa' COOH, are only a little less acid than acetic acid itself, because nearly all the effect of such larger radicals as they contain is already exerted by the methyl group.

But if a hydroxy! group be substituted, as in glyeolic acid, CHaOH-COOH, or a chlorine atom, as in monochloracetic acid, CH2C1- COOH, the acidity is greatly increased, while the compound trichloracetic acid, CCls- COOH, is a strong acid. On the other hand, aminoacetic acid, CHa'NHa-COOH, is scarcely acid at all. The effect of introducing a carboxyl group in place of a methyl group into any paraffine hydrocarbon, regardless of its constitution, e.g. CHa-CHa-^CHa-COOH, is to diminish the heat of combustion of the molecule almost exactly 157 calories ; but the conversion of acetic acid into oxalic acid, CHa'COOH-*- COOH -COOH, structurally an identical change, diminishes the molecular heat of combustion only 147 calories. "^ In both these instances it is certain that the nature of the influence of the radicals consisting of carbon and hydrogen exclusively is nearly independ- ent of their size and configuration. Any other group, however, by its presence at once modifies the nature of the case, though un- concerned in the process or property. Since it can be shown that such effects, like the difference between monochloracetic acid and

trichloracetic acid, depend upon the number of such foreign groups and their arrangement/ it is evident that the hydrocarbon radicals have a constancy of influence upon the prop- erties of the molecule which is not shared by other radicals. The indifference of effect of hydrogen and carbon, when linked to carbon, upon the properties of the molecule is undoubtedly a principal cause of the stability of complex organic substances. Through this peculiar- ity of the two elements the integrity of the valence energy of carbon is preserved, and the long carbon chains are stable. Whenever the molecule becomes overloaded with radi- cals of other kinds the strength of the tie between carbon atoms diminishes and the compound becomes unstable. The proper- ties of the carbohydrates, which will be later discussed, admirably illustrate such instabil- ity. In short, organic compounds are in some respects properly to be regarded as compounds of carbon and hydrogen jointly, for it is not the properties of carbon alone, but those of carbon and hydrogen together which chiefly make them possible.

' Henderson, Journal of Physical Chemistry, IX, 40, 1905 ; Proceedings of the American Academy of Arts and Sciences, XLII, 639, 1907 ; Zeitschrift fiir Physikalische Chemie, LX, In the more complex substances, such as the various ring systems of organic chemistry, it is not possible to discuss such problems of molecular mechanics. There too, however, hydrogen predominates over all other elements except carbon, and that may well be taken as a sufficient indication of its continued im- portance.

All of these considerations taken together suffice, I believe, to prove, or at least to make it exceedingly probable, that organic chemistry is in truth a unique field, and that no other elements can form compounds in such variety, complexity, and number as carbon, hydrogen, and oxygen. At any rate there can be no possible doubt that the compounds of organic chemistry are in these respects very remark- able indeed, and that similar cases must be extreniely rare among all the possible systems of compounds made up of all the known elements.

It follows from the peculiarities just ex- plained that the first great factor in the com- plexity of living organisms as we know them, the complexity and variety of their chemical constituents, depends principally upon the nature of the elements which compose such substances, and is most probably a unique, certainly a very rare characteristic of matter. That the very elements which make up water and carbonic acid, and apparently they alone, should possess this wonderful property is, when taken together with the physical prop- erties of water and carbonic acid and their place in cosmic evolution as constituents of the atmosphere, a fact which cannot lightly be set aside.

Not less valuable for the organism than the multiplicity of organic substances, and the diversity of their properties, are the great variety of chemical changes which they can undergo, and that characteristic instability which renders such great complexity of chem- ical behavior easily attainable. In short, organic substances are uniquely fitted not only to provide complexity of structure to the organism, but also, through their instability and manifold transformations, to endow it with diverse chemical activities, with com- plexity of physiological function.

One factor in determining the complexity of chemical changes which organic chemical substances manifest is the enormous number of simple structural relationships which every substance bears to others. This may be readily illustrated by the formulas of some of the derivatives of propane which possess biological importance : — Of the above substances, acetone, lactic acid, glycerine, glycerine aldehyde, dioxyacetone, alanine, serine, and cysteine are of the great- est moment in physiological processes.

Such complexity of chemical relationships results automatically, so to speak, in a variety of chemical transformations. But the trans- formations are greatly facilitated by that characteristic instability of organic substances, which is perhaps the chief distinguishing fea- ture of their behavior. For example, many inorganic substances may be subjected to very high temperatures without undergoing chem- ical change, but there are hardly any organic substances which can survive such treatment. Organic substances are also peculiarly liable to modification from the action of light and air. These are, however, but rough indica- tions of instability, and a special case will help more clearly to define the real characteristic.

In accordance with the researches of Emil Fischer, the following constitution is ordina- rily assigned to glucose.^ 1 A discussion of this formula may be found in any text- book of organic chemistry. It must be noted that carbon atoms to which four different groups are attached are asymmetric, that is to say, they can exist in two forms which resemble each other as the right hand resembles the left (Pasteur, LeBel, van't Hoff). This characteristic results in further increase in the number and variety of organic compounds. It is therefore neces- sary, in writing the formula, to represent the form of the molecule as it exists in space (in three dimensions), and this is actually accom- plished by imagining the three-dimensional formula to be projected upon the paper so that when the hydrogen atom is written to the right of the carbon atom one asymmetric form of the latter is designated, when the hydrogen atom appears to the left, the other.

It has long been known that when glucose is dissolved in water its optical activity, as the power of a substance to rotate the plane of polarization of light is loosely termed, changes slowly for some time before reaching a constant value. Recently it has been shown that this phenomenon probably depends upon the existence in solution of three different forms of glucose, which pass freely into one another and ultimately attain a state of equilibrium.^

* This subject has been fully discussed by Hudson, Journal of the American Chemical Society, XXXII, 889, 1910. When to such, a solution of glucose a small quantity of alkali is added, certain remarkable further changes occur, as was first demon- strated by Lobry de Bruyn.^ These pro- cesses result in the formation of mannose and levulose, probably according to the accom- panying diagram. Moreover, like glucose, levulose and man- nose both exist in solution in three different forms, so that the resulting solution contains at least ten chemical individuals. But it is almost certain that other changes simultane-' ously occur and that the solution is actually still more complex, even from the outset.

Upon a continued increase of alkalinity, or even slowly under the original conditions, a multitude of other changes set in. These * See the numerous papers by Lobry de Bruyn and Al- berda van Eckenstein in Recueil des Travavx Chimiques des Pays Bos, XIV-XIX. are known sometimes to lead to the formation of lactic acid, CHg-CHOH-COOH, methyl- glyoxal, CHs-CO'CHO, and formaldehyde, H • CHO. Further, as Neff has shown,^ many other substances may also be formed. Such bodies chiefly belong to the class of oxyacids. It is also certain that a great variety of other simple sugars resembling glucose, levulose,

and mannose are produced, and, all told, the constituents of such a solution probably number at least two hundred, all produced from glucose alone, under the influence of a slight excess of hydroxyl ions. Among these substances the greatest diversity of chemical behavior is to be distinguished. Alcohol, aldehyde and ketone, and acid radicals occur in great profusion and variety of combinations ; compounds possessing forked chains are pres- ent ; and double bonds between carbon atoms add to the complexity. Moreover, all these substances themselves possess great chemical activity.

A single case may perhaps illustrate this point. It has been shown by Windaus and Knoop ^ that in such solutions, in the presence of ammonia, one molecule of methyl glyoxal, one of formaldehyde, and two of amihonia unite to form the cyclic compound, methyl imidazol, a substance related to histidine, the latter being an important constituent of the protein molecule : — The instability of glucose and of all the simple sugars is indeed exceptional in char- acter, and the resulting processes are perhaps far more intricate and numerous than in any other similar case. However, this very case is of exceptional physiological importance, be- cause carbohydrates are the direct result of that synthetic action of chlorophyll,

which is the source of all organic substances and of all the energy of the organic cycle in plants and animals. Carbohydrates, more- over, are the chief constituents of plants and the chief food of animals. Turning to this synthesis of carbohydrate in the plant, we find much that is important in the present study. The details of the chemical transformation by which water and carbonic acid and solar energy are changed to sugars and oxygen still remain unknown, in spite of many careful investigations. But, at all events, it is possible to see that two things must somehow be done in order to accomplish the synthesis : —

(1) Carbonic acid and water must be re- duced. That is to say, oxygen must be sepa- rated from both of these compounds iSo that free valences may exist to unite carbon and liydrogen in one molecule, and so that, further, the relative proportions of the three elements may become what they are in the simple carbohydrates, C:H:0= 1:2:1. (2) Somehow individual carbon atoms must be joined together until there are six in each molecule, where formerly there was but one.

Theoretically, it might be possible by reduc- tion to form from carbon dioxide and water without union of carbon atoms the following substances: carbon monoxide, CO; formic acid, H-COOH; formaldehyde, H-CHO; methyl alcohol, CH3OH; and methane, CH4. Of such reductions the formation of formic acid, formaldehyde, and carbon monoxide has been directly realized by laboratory experiment.^ The most familiar theory of the formation of carbohydrates in the leaf is that of von Baeyer, which assumes a polymerization of one of the above substances, formaldehyde, leading directly to the formation of sugar, according to the reaction

This process also has been carried out ex- perimentally. Indeed, as a result of the investigations of Butlerow, O. Loew, and E. Fischer, it is known that in distinctly alka- hne solutions formaldehyde spontaneously goes over into a mixture of sugars which resemble glucose. Moreover, such a solution is un- questionably made up of much the same variety of substances as a glucose solution which has been subjected to the action of alkali in the same concentration.

As stated above, it is evident that if any- thing is to be done chemically with a mix- ture of carbon dioxide and water, oxygen must be split oflF from both carbon and hydrogen so that they may enter into the same mole- cule. If this chemical change, which, to be sure, is no easy one in the laboratory, be ac- complished, formaldehyde among other things results ; and in alkaline solution formaldehyde produces carbohydrates and leads to that amazing tangle of substances and reactions, whose nature has been briefly indicated above. In short, the one chemical 'process which is open, if any transformations whatever are to be accomplished with carbonic acid and water, leads directly and to all appearances necessarily to the greatest complexity that has been found in any one chemical process; to a system made up of possibly two hundred substances or more, most of which possess very great chemical activity.

It must not be forgotten that the share of the organism in such processes is also impor- tant. If possible, the smoothness with which chemical reactions are carried out in the leaf, perhaps quite without any stop at the formal- dehyde stage, and the certainty with which definite substances in large amounts instead of mixtures in very small amounts are pro- duced, seem more remarkable than the under- lying chemical facts. Needless to say, one great factor in such processes is the action of enzymes. Otherwise we are at a loss for a description of the ways and means by which the organism operates, though the brilliant studies of chlorophyll which have recently been carried out by Willstatter promise great achievements in the future.^

The underlying chemical facts, however, remain ; carbohydrates are among the natural products of carbon dioxide and water; they manifest in solution, especially in such con- ditions as obtain in protoplasm or the ocean,^ unparalleled instability and variety of reac- tions; and they produce spontaneously an enormous number of very active chemical substances. It is easy to see that, given ' See his many papers of recent years in "Liebig's Annal- len."