Popular Science Monthly/Volume 82/June 1913/The Evidence of Inorganic Evolution

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WHEN we consider the marked resemblances and striking interrelations of the elements as expressed by the Periodic System, the conviction grows more and more strongly upon us that this system is the external expression of a fundamental process in nature, to which are due the general properties, as well as the individual characteristics, of the elements. On the present occasion I shall endeavor to point out that between the Periodic classification and the ordinary zoological classification, such analogies exist as tend to indicate an identity in fundamental principle. We shall then consider some of the phenomena which are at the foundation of the law of organic evolution, and here, too, we shall find among the elements conditions exactly corresponding.

A Family of the Elements Compared with a Homologous Series

Before proceeding farther, however, it is of interest to note the similarities which exist between a family of the elements and a homologous series of organic compounds. For the purpose of this comparison it is most useful to select the homologous series of fatty acids, . If we should arrange the normal acids of this series in order of molecular weight, we should find that between such a series and a family of the elements there exist certain close analogies, which are tabulated below in parallel columns.

Fatty Acids () Family of the Elements
1. There is a constant difference in molecular weight between consecutive members of 14, due to the constant group difference . 1. There is a fairly constant difference in atomic weight between consecutive elements of the same family of about 45, except between the first and second (and in some cases between the second and third), where it is about 16.
2. The first member of the series, formic acid, differs somewhat in properties from the other members of this homologous series. Thus, it manifests the characteristics of an aldehyde, reducing ammoniacal solutions of silver nitrate, etc. It has no corresponding chloride or anhydride, is readily decomposed into and , etc. 2. The first member in each family of the elements differs somewhat from the other members. Thus, lithium differs from the other elements of its family in forming an almost insoluble carbonate and phosphate. Oxygen, again, differs from sulphur, selenium and tellurium in that its hydride is a colorless and odorless liquid, while those of the others are gases of disagreeable odor; in that it is seldom, if ever, more than divalent; in being gaseous under ordinary conditions of temperature and pressure, etc.
3. Several compounds of this series, which theoretically may exist, are unknown. Thus, between arachidic and behenic acids there is no acid corresponding to . Between behenic and lignoceric there is none corresponding to . Similarly, several acids are missing between cerotic and melissic. 3. Many elements which theory predicts should exist are unknown in the Periodic Table. Thus, elements are missing between silver and gold, between cadmium and mercury, etc.
4. The vacant places are all found in the lower part of the series, i. e., among the heaviest molecules. 4. The vacant places all occur in the lower part of the Periodic Table, i. e., among the heaviest atoms. The first four periods are complete (excepting the manganese family). In the last three periods many empty places appear.
5. Isomeric forms occur in the series, e. g., butyric and isobutyric, caproic and isobutyl acetic acids. 5. Allotropic forms occur in several of the families, e. g., the various forms of phosphorus, of sulphur, of carbon.
6. In a homologous series in general, the melting points, boiling points and specific gravities change uniformly and progressively with increase in molecular weight. In this particular series (considering, as before, only the acids with normal structure) the boiling points and specific gravities show this progressive change, and the melting points do also from caprylic acid on.[1] 6. Generally speaking, the melting points, boiling points and specific gravities change progressively and uniformly in each family of the elements with increase in atomic weight.
7. The acidity decreases with increasing molecular weight. 7. The oxides of the elements become successively less acidic (or more basic) in each family with increasing atomic weight.

The above relations show that a family of the elements possesses all the characteristics of a homologous series. There is evidently some identity of principle in the two things compared. We know that in the one case there is in the whole series a common plan of molecular structure, the differences in the structures of the successive normal acids being due to the constant and progressive addition of the same group of atoms, ; and hence it seems reasonable to suppose that there is likewise in each family a common plan of atomic structure,[2] to which are due the properties common to a family.

The Arguments from Classification

The fact that the groups of organisms fall naturally into a certain classification is in itself evidence of their origin by evolution.[3] Now, the most salient characteristic of this classification is a division into groups, and a subordination of groups within groups.

There is a breaking up into groups and sub-groups, and sub-sub-groups, which do not admit of being placed in serial order, but only in divergent and re-divergent order. . . . The Alliances are subdivided into Orders, and these into Genera, and these into Species.[4]. . . The conception finally arrived at, is, that of certain great subkingdoms, very widely divergent, each made up of classes much less widely divergent, severally containing orders still less divergent, and so on with genera and species.[5]

If we examine the characteristics of the Periodic classification, we shall find there the same peculiarities as have been observed in zoological classifications. Thus, there are the nine groups of elements, each quite distinct from the others, and each, as we have shown, very probably having a distinct plan of atomic structure common to all the members of the group. These nine groups correspond to the twelve phyla of organisms. Each group, again, is divided into two families, corresponding to the classes into which organic phyla are divided. That we have no further subdivisions corresponding to those in the organic classification is doubtless due to the circumstance that the number of elements is extremely small as compared with the number of species of animals. When we remember that even with this small number of elements, the Periodic classification presents many irregularities—as forcing into the same family elements with widely different properties (e.g., the copper family); creating a group of "transitional elements" different in the principle of its arrangement from the other groups; the breaking of the periodic sequence by argon, which is greater in atomic weight than potassium, yet precedes it in the series, and by tellurium, which bears a similar relation to iodine; and the irregularities presented by the rare earths—when these facts are considered, it can scarcely be doubted that if the number of the elements were at all comparable to that of organic species, the classification of the elements would necessarily present a subdivision of group within group as extensive, perhaps, as that found among organisms. Moreover, the periodic relation would probably be largely obscured by the great number of its irregularities and contradictions.

Since the classification into which organisms are naturally arranged, of group subordinated to group, is regarded as an indication of evolution, as previously stated; the fact that a similar arrangement is found in the classification of the elements suggests (when we consider also the other evidence to be presented) that we may regard the latter system in the same light.

Another peculiarity of organic classification, which, as shown by Spencer, is important because of its indication of evolution, is the variable degree of differentiation between corresponding groups and subgroups.

. . . The successively subordinate classes, orders, genera and species, into which zoologists and botanists segregate animals and plants have not, in reality, those definite values conventionally given to them. There are well-marked species, and species so imperfectly defined that certain systematists regard them as varieties. Between genera, strong contrasts exist in many cases; and in other cases, contrasts so much less decided as to leave it doubtful whether they constitute generic distinctions. So, too, it is with orders and classes; in some of which there have been introduced intermediate sub-divisions having no equivalents in others. Even of the sub-kingdoms the same truth holds. The contrast between the Molluscoida and the Mollusca is far less than between the Mollusca and the Annulosa, and there are naturalists who think that the vertebrata are so much more widely separated from the other subkingdoms, than these are from one another, that the Vertebrata should have a classificatory value equal to that of all the other subkingdoms taken together.[6]

Although at first thought this peculiarity may not seem to be of much importance, yet Spencer showed, by comparison with the case of languages, in which exactly analogous characteristics are observable, and in which evolution is known to have taken place, that it is an additional indication of evolution.[7]

If, then, the classification of organisms results in several orders of assemblages, such that assemblages of the same order are but indefinitely equivalent; and if, where evolution is known to have taken place, there have arisen assemblages between which the equivalence is similarly indefinite; there is additional reason for inferring that organisms are products of evolution.[8]

It will be evident that these observations concerning the organic classification apply with equal force to the Periodic classification. For instance, the elements of the alkaline earth family are not as sharply separated from those of the alkali family as they are from the inert gases or the halogens, and similar remarks apply to the other families. Within each group, too, the extent to which the two families comprising it differ from each other varies in the different cases. Thus, the elements of the chromium family are not as sharply distinguished from those of the oxygen family as the members of the copper family are from the alkalies. In the case of the elements, as in that of the organisms, the various groups and sub-groups differ from each other in the extent to which they are distinct from corresponding groups and sub-groups; and since in the latter instance, as we have seen, this peculiarity affords an additional indication of evolution, we have reason (when we consider also the other evidence) for so regarding it in the case of the elements also.

One other analogy demands recognition. Although, as previously stated, the phyla of organisms differ widely from each other, yet animals belonging to different phyla often show marked resemblances to each other in particular features. This phenomenon is a consequence of "the identity of plan, under the most diverse conditions of organization and habits of life (which prevails) not only among animals of the same group, but also between those of different groups."[9] For instance, regarding the affinities of the Rotifera (Phylum Trochelminthes) Parker and Haswell[10] state that

Their general resemblance to the free-swimming larvae of Annelids is extremely close. . . . The excretory organs recall those of Platyhelminthes, and also resemble the provisional nephridia or head-kidneys of Annulate larvæ.

Resemblances are also noted between the Class Gephyrea (Phylum Annulata) and Phoronis (Phylum Molluscoida).[11] The Crustacea (Phylum Arthropoda) "belong to the same general type of organization as the articulated worms [Phylum Annulata]."[12] Of the Phylum Mollusca it is stated that

The Mollusca. . . form an extremely well defined phylum, none of the adult members of which approach the lower groups of animals in any marked degree. There are, however, clear indications of affinity with "worms," especially in the frequent occurrence of a trochosphere stage in development, in the presence of nephridia, and in the occurrence, in Amphineura and some of the lower Gastropods, of a ladder-like nervous system resembling that of some Turbellaria and of the most worm-like of Arthropods, Peripatus. Rhodope, moreover, shows certain affinities with flat worms.[13]

Similarities are also pointed out between the sponges (Phylum Porifera) and the Cœlenterata.[14]

Corresponding to these counter-resemblances in structure among organisms, we have counter-resemblances in properties among the elements. Thus, mercury (Group II.) resembles copper (Group I.) in that both form two series of compounds, monovalent and divalent respectively, both form halides insoluble in water and decomposed by light, etc. Aluminum (Group III.) is similar to chromium (Group VI.) in that the hydroxides on heating give the oxides and , respectively, in that they form no stable sulphide or carbonate, etc. Thallium (Group III.) resembles, on the one hand, lead (Group IV.) in its metallic properties, in forming a chloride with properties similar to those of lead chloride, while, on the other hand, it resembles the lies (Group I.) in forming a hydroxide which is soluble in water and strongly alkaline in reaction.

Other examples, concerning which it is unnecessary to enter into detail, are the resemblances between phosphorus and sulphur; between beryllium and aluminum; between manganese and chromium; between boron, carbon and silicon; between gold and the platinum metals. It will be observed from the zoological examples above cited that the members of a phylum, while showing a greater or less similarity to each other, will often markedly resemble members of different phyla. The examples I have given show that a similar phenomenon is often characteristic of the elements of a family—the elements compared are in most cases similar to the other elements of the same family, while having at the same time the points of resemblance with each other described; and since the relationships referred to between distinct groups of organisms are believed to indicate a common origin, we may, perhaps, consider the analogous phenomena among the elements as of the same import.

Did space permit, other analogies might be pointed out between the Periodic and the zoological classifications; but enough has already been indicated to show that the Periodic classification possesses the main characteristic features of the zoological classification.[15] Now, the fact that these characteristics of the latter system are in themselves an indication of organic evolution suggests that the Periodic classification may be regarded in the same light, as I have already indicated. This suggestion is strengthened by the further evidence now to be considered.

The Homologue of the Embryological Evidence; the Phenomena of Radioactivity

The study of comparative embryology has brought to light certain facts which constitute important evidence of organic evolution; for many of the higher animals, in their immature forms, pass through stages in which they resemble more or less the adult forms of other animals, lower in the scale of differentiation. Moreover, animals of distinct but related species, in the progress of their development, often show marked similarities of structure. Von Baer

found that in its earliest stage, every organism has the greatest number of characters in common with all other organisms in their earliest stages; that at a stage somewhat later, its structure is like the structure displayed at corresponding phases by a less extensive multitude of organisms; that at each subsequent stage, traits are acquired which successively distinguish the developing embryo from groups of embryos that it previously resembled—thus step by step diminishing the class of embryos which it still resembles; and that thus the class of similar forms is finally narrowed to the species of which it is a member.[16]

This statement is now known to be too broad, but is true in general principle.[17] Again,

The embryos of the most distinct species belonging to the same class are closely similar, but become, when fully developed, widely dissimilar.[18]

To cite a few examples: The human embryo, at one stage of its development, possesses the rudiments of gill arches and gill clefts. The larva? of most insects, no matter how diverse, pass through a wormlike stage.

The larvæ of most crustaceans, at corresponding stages of development, closely resemble each other, however different the adults may become, and so it is with very many other animals.[19]

It is with the latter of the two embryological peculiarities mentioned above, viz., the resemblances between the embryos of different related species, that we are at present concernd.

In the radioactive transformations, we have not the advantage of witnessing a building up of elements from simple to complex forms, as in the process of embryology we observe the formation of complicated organisms from the egg. But, what is almost as good, we observe a devolution of elements, from complex forms to simpler. In the course of their disintegration, the three distinct elements, radium, thorium and actinium give rise to products (i. e., elements) which have very similar properties.

The substances thorium, radium and actinium exhibit many interesting points of similarity in the course of their transformation. Each gives rise to an emanation whose life is short compared with that of the primary element itself. Such experiments as have yet been made, indicate that these emanations have no definite.combining properties, but belong apparently to the helium-argon group of inert gases. In each case, the emanation gives rise to a non-volatile substance which is deposited on the surface of bodies and is concentrated on the negative electrode in an electric field. The changes in these active deposits are also very similar, for each gives rise to a rayless product, followed by a product which emits all three types of rays. In each case, also, the rayless product has a longer period, or, in other words, is a more stable substance than the ray product which results from its transformation.

The disintegration of the corresponding products, thorium B, actinium B and radium C is of a more violent character than is observed in the other products, for not only is an a particle expelled at a greater speed, but a particle is also thrown off at great velocity. After this violent explosion within the atom, the resulting atomic system sinks into a more permanent state of equilibrium, for the succeeding products thorium C and actinium C have not so far been detected by radioactive methods, while radium D is transformed at a very slow rate.

This similarity in the properties of the various families of products is too marked to be considered a mere coincidence, and indicates that there is some underlying law which governs the successive stages of the disintegration of all the radioelements.[20]

Starting out, therefore, with three distinct elements, we find them going through a process of change, in the course of which all three evolve products of very similar properties. If we regard this process of disintegration as in the main a reversal of a process of evolution which once took place, i. e., as a process of devolution, we may say, taking, for instance, RaG, ThD, and Act C as the starting points, that these elements commenced a career of spontaneous change, in the course of which transition products were produced which were quite similar to each other; but ultimately three distinct elements were generated. In the case of the elements, therefore, as in that of organisms, forms which were ultimately to be more or less dissimilar, passed, in the course of their development, through stages in which they closely resembled each other.

It is still more instructive if we consider the stages in the ontogeny of various animals in reverse order. We should then find, taking the Crustacea, for instance, as examples, that starting out with even the most diverse forms of these animals, and imagining them to go through the stages of their development in reverse order,[21] they would grow more and more similar as they approached the larval stage, and when they reached that condition, would be very much alike at corresponding stages of development; just as radium, thorium and actinium are much alike at corresponding stages in their degeneration.

Now, the significance of the embryological phenomena referred to is that these resemblances between animals of quite distinct groups are believed to indicate an ultimate common ancestry for the organisms so related; and since we have observed a condition which we may consider comparable to this among the elements, it seems probable that those radioactive elements which exhibit such close similarities as we have described as their disintegration progresses, originated by evolution from a primary simpler substance; it seems probable, that is, when taken together with the other evidences of evolution herein adduced.

Even if we disregard analogies, the fact that three distinct elements consistently show marked similarity in properties in the course of their disintegration would lead to the presumption that, if we could follow them back far enough, they might prove to be identical. This presumption is strengthened by the analogy we have considered.

It may be remarked that the changes occurring during radioactive disintegration are further similar to those which take place in ontogeny, in that in the former, as in the latter, the various stages are not permanent, but change continuously into other stages; and these changes are in both cases spontaneous, taking place without the aid of any external agency.

The Homologue of the Geological Record; Spectroscopic Evidence

Another source of evidence for the evolution of organisms is that derived from the study of paleontology; for the successive geological strata constitute a record of the organic forms which have successively inhabited the earth; a record which shows that in all the forms of life there is a considerable degree of continuity, and a (more or less) gradual transition from one form to another.

The homologue of this geological record in inorganic evolution is to be found in the series of stars arranged in order of decreasing temperature; for what the unknown cause of organic evolution has done for organisms, leaving the record in the geological formations, temperature (and perhaps other agencies) have done for the elements, leaving the record in stars of different heat intensities.

Lockyer has shown that the spectroscopic study of the stars, as carried on by himself and others, has revealed evidence of a very important kind for inorganic evolution. Here the results can only be briefly indicated.

As pointed out by Sir Norman Lockyer, the simplest elements appear first.

. . . In the hottest stars we are brought in the presence of a very small number of chemical elements. As we come down from the hottest stars to the cooler ones the number of spectral lines increases, and with the number of lines the number of chemical elements. . . . In the hottest stars of all we deal with a form of hydrogen which we do not know anything about here (but which we suppose to be due to the presence of a very high temperature) hydrogen as we know it, the eleveite gases, and magnesium and calcium in forms which are difficult to get here. . . . In the stars of the next lower temperature we find the existence of these substances continued in addition to the introduction of oxygen, nitrogen and carbon. In the next cooler stars we find silicium added; in the next we note the forms of iron, titanium, copper and manganese, which we can produce at the very highest temperature available in our laboratories; and it is only when we come to stars much cooler that we find the ordinary indications of iron, calcium and manganese and other metals. All these, therefore, seem to be forms produced by the running down of temperature. As certain new forms are introduced at each stage, so certain old forms disappear.[22]

The stellar evidence, like the geological record, is incomplete, because, as stated by Lockyer, of the very small range of the photographs of stellar spectra, and also because

It does not at all follow that the crucial lines of the various chemical substances will reveal themselves in that particular part of the spectrum which we can photograph.[23]

But whatever has been gleaned from the stellar evidence, though incomplete, is, like the information contained in the geological record, very significant in its indications of evolution.

The close analogies which we have shown to exist between the periodic and the zoological classifications would seem to point toward a fundamental identity of principle in these two systems. I have endeavored to show that there are in the inorganic world the exact homologues of some of the most important facts upon which the law of organic evolution rests, i. e., the evidence of the geological record and of the embryological resemblances; to emphasize the importance of the spectroscopic evidence; and to show that the Periodic classification is in its main aspects identical in its nature with the zoological classification. These facts tend to indicate that the groups of the elements correspond to the phyla of the organisms, in being the outward expression of a process of evolution. The periodicity in the arrangement of the elements is expressive of the fact that in each family there is the same plan of atomic structure, and a gradual and progressive change in this structure as we traverse the groups from the inert gases to the halogens. That it is an imperfect relationship is shown by its numerous contradictions, already mentioned. These facts, however, harmonize entirely with the evolutionary view, for zoological classifications show just such irregularities. Moreover, according to the evolutionary view, an element need not necessarily be smaller in atomic weight than the next in the same series. The evolutionary view is entirely compatible with those phenomena, which seem to be out of harmony with the Periodic classification.

If the species of organisms were few enough and their structure simple enough, it seems likely that it would be possible to select some common characteristic which would serve as a basis of periodicity corresponding to that in the elements. Conversely, as has already been indicated, if the number of the elements were at all comparable to that of organic species, it is probable that the Periodic relation would be largely obscured by the great number of its exceptions.

Without the knowledge of the fact of organic evolution, the arrangement of animals and plants into classes, with their numerous group resemblances and counter resemblances, must have seemed a purely arbitrary one, having no basis in nature.[24] Similarly, when we consider the characters of the elements of the same families, their close resemblances to each other, and their minor resemblances to members of other families, the irregularities of the Periodic classification, etc., it is evident that we can coordinate these seemingly contradictory phenomena into a coherent whole on the basis of the evolution of the elements. The extraordinary relations disclosed by the Periodic classification are the outward and manifest signs of the process to which atoms, like organisms, owe their individual natures. The process begun in the one (the atom) continues in the other (the organism).

  1. The physical constants here used (as well as the tabulation of the acids) are those given by Leathes, "The Fats," pp. 10-11. Other authors include several acids (e. g., pelargonic, undecylic) not mentioned by Leathes.
  2. For example, grouping of electrons, according to the well-known theory of J. J. Thomson.
  3. For a detailed discussion of this point, which can not be given here for lack of space, see Spencer, "Principles of Biology," Vol. I., pp. 356-359.
  4. Spencer, loc. cit., p. 297.
  5. Spencer, loc. cit., p. 358.
  6. Spencer, loc. cit., p. 361.
  7. For a detailed discussion of this point, see "Principles of Biology," I., 361-362.
  8. Spencer, loc. cit., p. 362.
  9. Claus and Sedgwick, "Zoology," Vol. I., p. 54.
  10. "Zoology," Vol. I., pp. 309-310.
  11. Parker and Haswell, loc. cit., p. 461. The zoological classification followed throughout is that given by these authors.
  12. Parker and Haswell, loc. cit., p. 556.
  13. Parker and Haswell, loc. cit., pp. 750-751.
  14. Parker and Haswell, loc. cit., pp. 215-216.
  15. The periodicity factor in the classification of the elements will be considered later (p. 97).
  16. Von Baer, quoted by Spencer, "Principles of Biology," Vol. I., p. 365.
  17. Cf. Spencer, loc. cit.
  18. "Origin of Species," Vol. II., Ch. XIV.
  19. Loc. cit.
  20. Rutherford, "Radioactive Transformations," pp. 169-170.
  21. Such a process need not be wholly imaginary, however; phenomena comparable to this are found in the instances of so-called retrograde development.
  22. Lockyer, "Inorganic Evolution as Studied by Spectrum Analysis," p. 159.
  23. Lockyer, loc. cit., p. 161.
  24. "The propinquity of descent—the only known cause of the similarity of organic beings—is the bond, hidden as it is by various degrees of modification, which is partly revealed to us by our classification." Darwin, quoted by Spencer, "Principles of Biology," Vol. I., p. 364.