Creation by Evolution/The Nature of Species

by John Walter Gregory

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THE NATURE OF SPECIES

By John Walter Gregory

Professor of Geology, University of Glasgow

The final test of the theory of organic evolution is whether the different kinds of animals and plants are fixed and unchangeable or whether one kind may through its posterity give rise to or pass into another kind, even though the passage may be so slow that the changes produced in the lifetime of one observer are slight. No highly specialized organism can be expected to develop into an altogether different organism; there is no chance, for example, that a humble-bee will give birth to an elephant, or that a club moss will develop into a fruit tree. Nor can any decisive verdict as to the natural evolution of new forms of animals or plants be given from a consideration of the various breeds of sheep or dogs or garden plants. New breeds can unquestionably be developed at the will of the breeder; but the fact that domesticated animals can be varied by breeding does not necessarily show that under natural conditions the progeny of one kind of elephant can become another kind, or that a certain sort of moss, if placed in a new environment, will become another kind of plant.

The work done by breeders shows the plasticity of living forms, and like plasticity is seen in animals and plants living under natural conditions, but the question is whether there are in the animal and vegetable kingdoms any well-established units between which distinctions—even comparatively slight distinctions—can be marked by boundaries that are impassable. Such units were once supposed to exist, and they were called species, a word meaning a kind. The name species was given under the impression that, though each kind so designated might vary to some extent, the variations were restricted within limits that could not be transgressed. The differences between members of the same species were regarded as individual variations. Slight differences between plants and animals of the same species were recognized by careful observers, and the abrupt changes seen in sports and monstrosities attracted the attention of the curious. Many early authors placed no limit on the extent to which such variations might occur. Bacon observed a fern growing out of a willow and, instead of explaining it as a natural graft due to a windblown spore caught in a crack, regarded it as an offshoot due to some injury or some special influence. He also suggested that the stump of a felled beech might put forth birch, it being "a tree of a smaller kind which needeth less nourishment." Thus, according to Bacon, a beech might be developed into a birch by an unfavourable environment.

The belief in the fixity of species arose in the generation after Bacon. Herbert Spencer^[1] attributed it to a literal acceptance of the Mosaic account of Creation, and it has often been credited to Milton, who relates, in Paradise Lost,^[2] how, on the sixth day, in accordance with the Divine command,

The earth obeyed, and, straight,
Opening her fertile womb, teemed at a birth,
Innumerous living creatures, perfect forms,
Limbed and full grown.

But, as Professor Poulton has remarked, the belief in the sudden appearance of animals in their present forms was due less to Milton than to the faith of his fellow Puritans in the verbal inspiration of the Bible.

The formal definition or establishment of species was practically begun by Milton's contemporary, John Ray, an Essex naturalist (1627-1705). Ray founded many of the species of the British flora and prepared the way for Linnaeus (1707-1778), whose system was based upon implicit faith in the immutability or fixity of species. Linnaeus declared, in a famous dictum, that "the number of species is as many as different forms were created at the beginning." In the following century Cuvier was equally positive. He believed that species are as distinct as the different makes of boots sent out from a factory. Darwin, on the contrary, called his epoch-making treatise "The Origin of Species," because he maintained that species pass into one another; and the doctrine that one species may be derived from an earlier allied species is the doctrine of organic evolution. He regarded the term species as one arbitrarily given, for convenience of designation, to a set of individuals closely resembling one another—as a term not essentially different from the term variety, which is given to less distinct and more fluctuating forms.

Darwin's theory was opposed to preconceived opinion. The species of the more highly organized animals and plants appear to be sharply separated. The differences between the African and Asiatic rhinoceros, between the African and the Indian elephant, between the one-humped and the two-humped camel, appear to be constant and absolute. Nevertheless, when such animals are examined carefully it is found that individuals in different herds of each kind show slight but significant differences. The giraffes, which were at first classified as one species, have been broken up into eleven subspecies; the African elephant has been found to include more than one species and several subspecies. Systematic botanists and zoologists have been divided into two schools — the "Splitters" and the "Lumpers." The "Splitters" establish species on differences which the "Lumpers" treat as mere individual and inconstant variations. Darwin represented Asa Gray, the famous American botanist, as a Splitter, and Sir Joseph Hooker, of Kew, as a Lumper. Herbert Spencer, in 1852, estimated that the number of species must amount to at least ten millions. The Splitters would multiply that number many times.

There has been no agreement as to what characteristics should be regarded as of specific rank—that is, as sufficient to justify a naturalist in founding a species—and as to what are of a lower systematic value. For example, there has been a long-continued controversy whether man is one species or whether the European, the Negro, and the Mongolian are distinct species. This difficulty has been partly overcome in practice by the introduction of minor units of classification, which have been called subspecies, and the subspecies have been divided into varieties, and these into subvarieties, and these in turn into races and subraces. The divisions are thus numerous, and the grounds for them are indefinite. Different groups of plants and animals have different grades of specific subdivision, according to the abundance of their members, or their variability, or the attention they have attracted. Thus the Flora of France uses in some genera six subdivisions lower than the genus. British botanists adopt more subdivisions of species in roses and brambles than in less variable plants. Some species of British land snails, such as the common Helix nemoralis, have undergone indefinite subdivision.

The extent to which experts differ as to whether certain variations are distinctive of species, varieties, or races shows that there are no such fixed limits to species as the pre-Darwinian naturalists believed. A species is an expression of opinion, not of fact.

Persistent efforts were made to delimit species by the sterility of hybrids. The fact that the mule is sterile was set up as proof that the horse and the donkey are different species. But the rule that hybrids are sterile is subject to many exceptions; thus the rabbit and the hare have a fertile hybrid known as Lepus darwinii, and so have the common and the Chinese goose, which are classified as unquestionably distinct species. Conversely, crosses between many domesticated varieties of plants are sterile.

The fact that a species is an arbitrary and not a well-defined natural unit is further shown by variations of organisms from the standard types. Herbert Spencer, in 1852,^[3] years before the publication of Darwin's Origin of Species, claimed that it had already been proved that “any existing species—animal or vegetable—when placed under conditions different from its previous ones, immediately begins to undergo certain changes of structure fitting it for the new conditions. They [the supporters of the theory of natural development] can show that in cultivated plants, in domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded. They can show that it is a matter of dispute whether some of these modified forms are varieties or separate species.”

Some variations are obviously due to the influence of environment and of mode of living—of daily work. Variations that are successive and cumulative in time have been called mutations (Waagen, 1868), though that term has been

Fig. 1.—Shell of Micraster corbovis.

“A. W. Rowe, a physician of Margate, devoted his holidays to collecting heart-urchins (Micrasters), from the Chalk of England, foot by foot. He was able to show that what appeared to be a distinct species found at the bottom of the Chalk gradually changed into different species found at the top. This change is almost imperceptible, but it can be traced in every part of the fossil shell, and it takes place in the same way in all parts of the country. Here is an example of evolution caught in the act. If we were to take a set of photographs of these fossils from the base of the series to the top, and copy them on a cinematograph film, we could see evolution taking place before our eyes.”—Dr. F. A. Bather (Creation by Evolution).

“The members of one single genus of sea-urchins would have to have been wiped out and replaced by barely distinguishable successors some dozens of times during the course of the deposition of the English chalk—if their fossils do not show descent each from a previous and slightly different ancestor.”—J. B. S. Haldane.

There is in nature no inseparable dividing line between different organic forms; for some forms there is such a gradual shading of one into the other, that it is impossible to tell where one ends and the other begins. Species are not primordial forms, fixed and impassable from the beginning; they are in fact constantly changing and new species or forms are constantly appearing.

used later (de Vries, 1901) in a nearly opposite sense—that is, to denote sudden variations, or sports.

Herbert Spencer remarked: “Those who reject the theory of evolution as not adequately supported by the facts accept instead a theory which is supported by no facts at all.” The development of specific differences—differences marking species—by gradual change from generation to generation has been well established by collecting fossils from successive layers in a series of deposits and comparing each fossil with its predecessors and successors.

Fig. 2.—Sea-urchins formed in the English Chalk.

The sea-urchins in the English Chalk that belong to the extinct genus Micraster (Fig. 1) provide a convenient illustration of the evidence thus obtained as to the actuality and nature of evolution. This sea-urchin is a common and well-preserved fossil, and it shows variations that might be regarded as distinctive of two or more species. The late Dr. A. W. Rowe, of Margate,^[4] collected 2,000 specimens of Micraster and carefully recorded for each specimen its level in the Chalk. They show gradual variations (Fig. 2) as they are followed upward through the Chalk. They are of four chief shapes, all of which have come from the earlier form M. corbovis. One kind retained the flat top and elongated form of the typical corbovis; another became arched above; a third developed a prominent posterior ridge; and a fourth became conical or pyramidal. These four series may all be included in the same species, being, according to Poulton’s term, epigonic—that is, descended from one pair of ancestors; or each series would answer to Darwin's conception of a species, for each showed an appreciable difference and persisted for a considerable period. Rowe’s collection, however, showed that though the different shapes were developed by slow change, they were not developed by continuous divergence from the original type. The contemporaries of the early corbovis included dome-shaped and flat varieties. The forms of Micraster found at each horizon or each level in the Chalk show variations that were probably dependent on the nature of the sea floor and on the movement of the sea water. The evolution of the four chief varieties of Micraster in the Upper Chalk was not due to steadily progressive variation from the ancestral type but proceeded by innumerable minor irregular variations, the effect of the environment acting upon successive multitudes of Micraster. The evolution was doubtless due to syngamy or interbreeding under natural conditions, with the encouragement by the environment of the useful modifications. It did not proceed along lines of continuous and steadily diverging variation, but along several parallel lines, from each of which there were variations that would overlap those from the next line. Dr. Rowe, in his diagram showing the evolution of the Chalk Micraster, used the plan of branches divergent like the twigs of a tree. The main types were established by parallel lines of descent, the variations from each line being radial in all directions and overlapping those from adjacent lines. This conception agrees with that stated by Poulton when, in closing a discussion at the Entomological Society on “What is a Species?” he remarked^[5] that he had never conceived of the origin of a species from one ancestral pair, but always from the change of masses rather than of individuals. He added that it was “the splitting of the single community into separate subcommunities which was the foundation of the process.”

These subcommunities were the kinds of groups for which, in 1896, I suggested the term circulus. In a catalogue of the Jurassic Bryozoa, or “moss-animals,” in the British Museum I pointed out that the term species is inappropriate to these groups, because they are not separated by definite boundaries and were not developed by continued divergence into isolated assemblages. The term circulus was suggested from the analogy between these groups and the knots of people who collected around the speakers in the Roman Forum. Each knot would be crowded near the centre and looser on the margin, whence people would frequently pass to an adjacent circulus.

The term circulus was also used in 1900 in a work^[6] describing a large collection of fossil corals from the Jurassic deposits of Cutch, in western India. Most of the corals came from one reef, which was especially rich in the simple coral Montlivaltia, of which there were more than 2,000 specimens. Each circulus of these Indian Montlivaltia shows variations as great as those representing species among the corresponding European corals. One of the flat corals, named M. frustriformis because it is shaped like the frustrum of a cone, has fifteen European analogues; a taller hornshaped form, M. cornutiformis, corresponds to twenty European species, and M. kachensis to eleven. In the European Jurassic rocks many of the coral reefs are isolated, occurring at places where warm currents from the south raised the temperature of the water. Most of these coral reefs have disappeared, and thus, though the development of the European Montlivaltia may have been continuous, it appears discontinuous owing to the imperfection of the geological record. In Europe it is therefore convenient to treat the separate groups as species; but in India, where a large number of corals were collected from one area of slightly undulating sea floor, the variation is continuous. The groups of corals are knots of individuals, or circuli.

The opportunities for tracing progressive evolutionary series of fossils in the field are not numerous; such series can be found only where thick deposits have been laid down continuously under the same geographical conditions, so that for a long period forms were deposited one above another and the intermediate forms were preserved in their right order. The Chalk is one of the formations to which this method can be applied. It is a soft, earthy limestone, in places a thousand feet thick, and was laid down as an almost continuous deposit of limy mud, so that the fossils are perfectly preserved and easily extracted; and owing to the many uses of chalk large exposures are available in inland quarries as well as in continuous sections in sea cliffs. Other groups of Chalk fossils show the same continuous evolution as Micraster. The process has been demonstrated in other formations, as by Hyatt and others for the ammonites, by Carruthers for a Carboniferous coral, by Schuchert and H. Walker for the brachiopods.

Opportunities for the study of contemporary variations are more common, both with fossils and with living animals or plants. Where organisms live in large numbers under similar conditions the attempt to divide species becomes practi

Shells of Paludina (after Neumayr).

A graduated series of forms showing one of the strongest evidences for evolution. These changes certainly show one of two things: either each change constitutes an independent separate act of special creation, or one form gradually alters and merges into another.

Is it more reasonable to think the forms would have been created to look as if they were related—as if one were the slightly altered offspring of another; as if they were a connected series—or that such a complete sequence can only mean relationship; that similarity of structure implies a common origin?

“The essence of evolution is unbroken sequence.” Editor.

The extremes (I, IX) would constitute separate species were the means (II–X) not living contemporaneously.

The idea of evolution leaps to the eye when we look at a series like this.”

J. Arthur Thomson.

cally impossible. Shell-fish which are so scarce that a museum collection contains only a few representatives are easily divided into species or varieties; but shell-fish such as oysters, which live together in multitudes, are indefinite and uncertain as to species. The same difficulty has been observed with the sea-butterflies, or pteropods, which live in swarms on the surface of the sea and form a large part of the food of whales. In groups where specific variation was slow, or the members were few, or the fossil remains are rare, the differences are so well marked that the delimitation of species presents no difficulty. Organisms, however, that live together in vast numbers and under similar conditions show continuous variation, and though the individuals may be massed around certain centres, the groups grade into one another.

The arrangement of such groups into circuli instead of into species is a fulfillment of Huxley's prediction in 1880 that “The suggestion that it may be as well to give up the attempt to define species and to content oneself with recording the varieties . . . which accompany a definable type ... in the geographical district in which the latter is indigenous may be regarded as revolutionary; but I am inclined to think that sooner or later we shall have to adopt it.”

The artificial nature of species has been generally recognized by working naturalists; but the term species is still retained. Sir Ray Lankester, with his logical consistency, recommends that it should be abandoned; but it has been maintained from tradition and convenience. The abstracts of the papers contributed by Prof. H. L. Hawkins and Dr. A. E. Trueman to a recent British Association discussion on the “Conception of Species” show that their idea of a species is that of the circulus; and so also is the “species-group” of Dr. Bolton among fossil beetles. It is fully time that the term species should be less frequently used, as it is apt to mislead. It was based on die belief that species are fixed and immutable, and its use encourages that belief. That “species are species” is a statement often made in support of the idea that naturalists in practice treat species as fixed.

The circulus, on the other hand, is a natural grouping, which adopts evolution as a fact and as achieved by slow variation in all the members of a group in various directions. Each circulus has what Bateson called a centre of organic stability, and most of its members tend to be near the centre so long as the condition remains the same. Wherever during the growth of the Cutch Montlivaltia the sediment deposited on the sea floor accumulated steadily, most of the corals would have the same ratio of diameter to height; but if it accumulated at one place more quickly than at others, the centre of organic stability at that place would be with corals that were higher in proportion to their width.

The circulus provides a nomenclature which is consistent with the view that evolution results from changes affecting the mass of individuals belonging to a group and which relieves the naturalist of worrying over, say, the number of species among British brambles. Discussions of such problems, except in so far as they stimulate close observation, are comparable in utility with the mediaeval arguments as to how many angels could stand on the point of a needle.

REFERENCES

• Bateson, W. The Study of Variation, Treated With Special Regard to Discontinuity in the Origin of Species. 1894.
• Bolton, H. Insects from the Coal Measures of Commentry. 56 pp., 3 pl. 1925.
• Gregory, J. W. Catalogue of Fossil Bryozoa in the Department of Geology, British Museum. The Jurassic Bryozoa, pp. 1, 22–28. 1896. Jurassic Fauna of Cutch, Vol. II, pt. 2. The Corals, pp. 17–23. 1900. Palaeontologia Indica, ser. IX.
• Poulton, E. B. President’s Address. “What is a Species?” Trans. Entom. Soc. London, pp. lxxvii–cxvi. 1903.
• Rowe, A. W. An Analysis of the Genus Micraster, as Determined by Rigid Zonal Collecting from the Zone of Rhynconella cuvieri to that of Micraster cor-anguinum. Quart. Jour. Geol. Soc, Vol. LV, pp. 494–547, pi. 35–39.
• Spencer, Herbert. The Development Hypothesis. Reprinted in Essays—Scientific, Political, and Speculative, I, 1868, 1854.
• The Conception of Species. A discussion at the British Association, Oxford, 1926. Rep. Brit. Assoc, 1926, pp. 356–357.

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“All students were so impressed with the belief in the reality and permanence of species that endless labour was bestowed on the attempt to distinguish them—a task whose hopelessness may be inferred from the fact that even in the well-known British flora one authority describes sixty-two species of brambles and roses and another of equal eminence only two species of the same group.”—A. R. Wallace.

“A species is supposed to be a group of individuals that closely resemble one another owing to their descent from common ancestors—a group that has become more or less sharply separated from all other coexisting species by the disappearance of intermediate forms.” “The more we study the animal and vegetable kingdoms . . . the more clearly is the fact impressed upon us that if we could have before us all past and present individuals we should find it impossible, except in an arbitrary manner, to arrange them in species at all, for each kind would be found to be connected with others by series of small gradations.”—Arthur Dendy, Outlines of Evolutionary Biology.

“The question what constitutes a species must be left to the judgment or fancy of the individual.”—H. S. Jennings.

Modern students of nature do not find, as Linnaeus stated about two hundred and fifty years ago: “There are as many different species of animals and plants on earth as there were different forms created in the beginning.”

1. In 1852, reprinted in Essays, vol. Ⅰ, p. 583, 1868.
2. Book Ⅶ, lines 387-500.
3. Reprinted in Essays, Vol. 1, pp. 379-80, 1868.
4. Quart. Jour. Geol. Soc., Vol. 55, 1899, pp. 494–547, pls. 35–9.
5. Trans. Ent. Soc. London, 1911, p. XVI.
6. Palaeontologia Indica, Ser. IX, vol, 2.