Popular Science Monthly/Volume 73/December 1908/Aspects of Modern Biology
| ASPECTS OF MODERN BIOLOGY[1] |
By Professor T. D. A. COCKERELL
UNIVERSITY OF COLORADO
DURING the latter part of August, last year, the International Zoological Congress met at Boston. This circumstance was not very widely heralded by the press, nor did it make an impression on the public mind at all comparable to that ordinarily produced by any serious crime. Nevertheless, it was an event of the first importance, this gathering of the zoological forces of all civilized countries to take stock of the progress of the science and exchange fraternal greetings.
To present any summary of the things said and done at that meeting is neither desirable nor possible at the present time; but it may be useful to consider what it all meant—where zoology now stands, and what it stands for.
Most typical, perhaps, of the whole trend of zoological thought was the address of Professor William Bateson, of Cambridge University. It dealt with the subject of genetics; the genesis of things, cells, individuals, species. It told of sequences actually observed rather than contemporaneous facts arranged in rows. The methods advocated were experimental, the range of investigation was the whole field of life.
At the same time, the geneticologist did not refuse to recognize the value of the other methods of research. Said Professor Bateson: "When morphology was a new idea, everything was sacrificed to its pursuit. Physiology, systematics, all were discarded as useless lumber. Let us not repeat that short-sighted mistake. In the wider survey which we are attempting we shall need all these things. If we are to understand rightly the phenomena of specific difference—to take that problem only—we shall be glad of anything that the systematist can tell us, and of many deductions of pure physiology."
The old natural history is having a new birth, with new hopes and aspirations, but with the same unity of interest and of purpose. With the growth of science, specialization was necessary and desirable. Yet as time went on and zoology not only grew apart from botany, but the various branches of zoology seemed to have different languages, it appeared as if a tower of Babel would result. Even the systems of nomenclature for genera and species, ostensibly the same throughout, came to differ appreciably in different departments; and the various specialists were so segregated that they were scarcely or not aware of the fact. The grouping was sometimes according to classes, orders and families; sometimes according to physiological or morphological aspects of things; but everywhere it seemed that narrowness increased, and that broad conceptions of the Darwinian type were fading away.
The new attitude typified by Professor Bateson not only unifies the branches of zoology, but makes zoology and botany one. Greedy for the results of special research in every department, it yet makes all serve a common end. So far from despising or discouraging the most minute enquiries in limited fields, it gives them all a new purpose and new meaning, as contributory to the philosophy of life, which is, indeed, the sum of all philosophy. We find ourselves at that meeting point of monism and dualism, of synthesis and analysis, where the electric spark of human understanding always has had and always will have its birth.
Professor Forel, of Switzerland, in a recently issued book, has called attention to the difference between mathematical and ordinary reasoning, including in the latter the methods necessarily employed in the biological sciences. In mathematics, we start with certain postulated facts, and given definite methods of procedure, climb up a ladder of argument, each part of which is supported by the one below. An error at any point vitiates the whole piece of work; while, if there is no error, the result is said to be demonstrated beyond dispute. Systems of logic have been constructed in the same manner, and such processes have found great favor with lawyers and theologians whose main purpose has been to support theories rather than ascertain the truth.
In the natural sciences, as in the every-day affairs of life, the method is entirely different. Desiring to determine the state of things at any point in time or space, we converge upon it all the pertinent evidence we can secure, and form a judgment upon the collection. We do not profess to exclude the possibility of error; rarely do we feel so well supplied with facts that others are not welcome. Those of us who have worked long among biological facts have so often made mistakes, or discovered the mistakes of others, that we have become somewhat more humble-minded and less assertive than we used to be. This humility, however, is coupled with a keen sense of the tremendous weight of evidence in favor of certain conclusions. We do not assert that we must be right, but we at least demand an equivalent load on the other side of the scales before changing our opinion: a demand not readily comprehended by those to whom our body of facts is invisible.
In Colorado to-day we find existing many millions of individuals of animals and plants, presenting extreme diversities of form, color and size, and distributed in certain particular ways. It is the business of the naturalist to find out the how and why of all this, so far as he can. Before he attempts to formulate general laws, he must collect his facts, and examine them in detail. He goes out, perhaps, and gathers a flower: let us say the blue flax, Linum lewisii. The very name tells him something of its history; it was called lewisii in honor of Meriwether Lewis, of the famous Lewis and Clark expedition. It has been known since 1814, and has been collected by many botanists. Turning to the published records, it appears that it has been found as far south as the mountains of Mexico; as far north as Alaska. It does not occur in the eastern part of the continent, north or south. Among the native plants of America it has only one close ally, a smaller plant called Linum pratense—we may call it the prairie flax—which occupies open ground east of the Rocky Mountains from British America to Texas and again appears in Arizona.
In Europe and Siberia, however, there are closely similar plants; so like our Linum lewisii that for many years our plant was not separated. Furthermore, there are various other species of Linum or flax in the Old World, some of them strikingly different from ours. These, some with blue, some with red flowers, are more closely related to one another than to the yellow-flowered flaxes, which have lately been placed in a separate genus.
From all these facts, it is permissible to assume (in the absence of contrary evidence) that the genus Linum, in the restricted sense, belongs especially to and probably originated in the temperate regions of the Old World. This opinion is fortified by the discovery of a species (Linum oligocenicum Conw.) in European amber of Tertiary age. We imagine, then, the true flaxes originating perhaps in central Europe or Asia, segregating into various distinct species, and finally, perhaps during the Miocene period, invading North America. From the present distribution of the plant, we should naturally infer that it came by way of Bering Strait, not across the Atlantic; and from its slight divergence from the Old World stock we should think of it as a comparatively recent immigrant. The prairie flax, occupying a lesser area, and not so similar to the Old World type, is regarded as an offshoot from Lewis's flax, adapted to life on the prairies, the former occupying the mountains.
Leaving the flax for the moment, our naturalist hunts about and picks up a small shining cylindrical shell known as Cochlicopa lubrica. This snail is distributed widely over the continent, from Canada to Alabama, and west to the Pacific coast region. It is very constant in its characters, but in a few states has given rise to a variety or closely allied form of larger size called morseana. There are no other American allies.
So far, there is no apparent clue to its history; but when we turn to the eastern hemisphere we find a very different state of affairs. In the regions surrounding the Mediterranean there are dozens of species of shells of the same general type, while C. lubrica itself is widely spread over the whole of Europe. Moreover, this species lubrica, so constant with us, is there much more variable, so that nine varieties have been found in the British Islands alone.
The presumption is, then, that the snail, like the flax, is of Old World origin, and represents a comparatively recent invasion from the ancient area of distribution. This is supported by the occurrence of allied but distinct genera in Europe and adjacent regions.
Further investigation reveals hundreds of other cases similar to those of the snail and the flax, and so it becomes more and more probable, finally practically certain, that we owe a considerable part of our fauna and flora to the immigration of animals and plants which has reached nearly their present condition on the other side of the world. We ourselves, of course, belong in this category.
Having arrived at this point of view, the subject must not be dropped, but should be attacked from another side. If America has been overrun by Old World types in comparatively recent times, it should be possible to get some idea of the time of these invasions by examining the fossils of various strata. Unfortunately, the paleontological record is very imperfect, but it yet yields facts of prime importance. We find that certain types, living in Colorado to-day, have lived here with only slight modifications for many thousands, perhaps some millions, of years. Others are totally absent, so far as our information goes, from the older Tertiary strata, but negative evidence of this kind must always be received with reservations. Others, to-day only found in Asia, Africa, Europe or South America, were conspicuous members of the Colorado biota. Here we find facts which throw doubt on some of our previous conclusions. The Equidæ, or horse family, have to-day numerous members in Africa and some in Asia, but none whatever in America. Yet we have evidence from the fossils that there were formerly horses in America, and that they actually evolved on this continent. The disease-carrying tsetse flies are to-day exclusively African and might well be thought a peculiar product of that continent, but a species has turned up in the Colorado Miocene! So with other cases, all tending to show that it is not safe to assume without question that the original center of a group is the region where it is now most abundant and varied. We do not thereupon decide that the evidence from present distribution is valueless; in many more instances it leads to exactly the same conclusions as might be derived from the fossils; but we recognize the importance of supplementing one kind of fact with another, and considering all together when forming conclusions. When using the paleontological evidence, we are struck by the differences between the fossils of successive strata and are always inclined to regard these as indicating widely different periods. Here the facts of present distribution serve to make us hesitate. Different altitudes, different soils, different conditions of moisture and so forth, produce to-day very distinct sets of animals and plants, even in the same immediate region. Or, if we are dealing with marine forms, a littoral and a deep-sea fauna of precisely the same age would be very different; indeed it is doubtless not an exaggeration to say that the present shallow-water fauna of the Atlantic coast resembles the shallow-water fauna of Middle Tertiary times much more than the deep-sea fauna of to-day. Considerations of this sort have led careful paleontologists to attempt to estimate the climatic and other conditions surrounding the subjects of their investigations; thus, for instance, Dr. Matthew, in discussing the Tertiary mammals of northeastern Colorado, concludes that one series represents a plains or prairie fauna, the other a forest one. These are not of the same age, but the difference between them is clearly to be attributed to environmental conditions as well as the lapse of time.
Thus in the course of our enquiry we come back to the modern biota, and find it necessary to ascertain as accurately as possible what conditions permit the existence and migration of the several species. Neither the blue flax nor the snail exists everywhere within the region which we said, in general terms, that they occupied. The flax occurs at various altitudes, up to 10,000 feet, but always in more or less open places, in dry or at least not very moist soil. The snail also lives at different altitudes, but in moist places under vegetation. Thus, although when plotted on a map the ranges of the two would appear to largely coincide, it is probable that they never, or almost never, actually exist together. While spreading over enormous areas, they have picked their way, as it were, from one suitable spot to another, showing thereby how closely they are dependent upon a particular set of conditions. In a general way, mountains may be said to favor the spread of both, and for either the desert is an impassable barrier.
When we have ascertained the necessary conditions of moisture, heat, light, etc., we have not nearly solved the problem. Very important, in nearly every case, is the living environment. In the case of the flax, we know that it is very injuriously affected by an orange rust (Uredo lini (Pers.) Schum.), which extends practically throughout its range. This rust infests not only the blue flax, but other species as well, including some of the yellow ones. Consequently, when two geographical groups of flax plants meet, whether they are of the same species or diverse, there is always a possibility that one will convey the rust to the other, supposing that they are not both already infested. The same sort of thing is true of diseases of animals, as many races of men have found to their destruction, upon mingling with the white man. The competition between allied species is thus often indirect, one destroying another by conveying to it some disease.
The flax is visited by various bees, which I have studied and recorded; these carry the pollen from flower to flower, and thus aid in pollination. Whether the necessary bees are always present, is not yet known; but their presence in numbers must be a favorable factor, and thus an important element in the living environment.
In the case of the snail, although it is so common, we know little or nothing about its natural enemies.
The more we study living creatures the more we become impressed by the complicated conditions necessary for the preservation of the higher forms, and the possibilities of local or complete extermination. As we determine these more accurately, we feel able to return to the fossils, and from them restore the past in much more detail than at first seemed possible. If a snail or a slug crossed from Asia to America we presume that it not only found continuous, or nearly continuous, land, but also that it did not traverse any desert. The path of migration of the blue flax was not, it is virtually certain, across a lowland region or swamp. Making all allowances for what are called accidental means of transportation, it ought to be possible to infer something about the pathway of a considerable number of species.
In all of these researches, success and failure are inextricably mixed, at least as regards the details. In no case can we gather all the pertinent facts; our knowledge of even the commonest species is very deficient. Yet, when all is taken together, we find ourselves like the man who said he lost on every job, but was able to make money because of the multitude of them. The number of known species, living and extinct, is enormous, and the data we have gathered, when suitably sorted and arrayed, will point to many definite conclusions. More especially is there reason to hope for good results to be derived from studies which past investigations have merely suggested and shown to be possible.
As a matter of history, as food for the imagination, it is interesting enough to watch and take part in the reconstruction of the past, especially when we are able to do this with a reasonable degree of completeness, as at Florissant in Colorado, or Œningen in Germany. Much more, however, may come of these investigations. The problems of evolution, the intricate questions of heredity and variation, may be answered in part by such means as I have described.
The experimentalists, represented by Bateson, De Vries, Tower, MacDougal, Davenport and many others, have ascertained that what appear to be new species or races may arise suddenly by a process termed mutation. It even appears that in certain cases this process may be brought about by artificial means, such as differences of humidity, or certain substances in solution, supplied at the proper moments. The obvious suggestion is, that species are more readily modified than is commonly admitted; and that in particular they are likely to be so modified on the borders of the territory they occupy, where they continually impinge on unaccustomed environments.
To show that this is possible is a most important step; but we still have to enquire how far has it actually occurred? In the case of our flax, we have an excellent example of the production of a new form on the periphery of the old, permitting expansion through modification; but only one such derivative seems to have been produced. In other instances, as the experimentalists have shown, the apparent instances are illusory the supposed geographical segregates being merely examples of a single type variously modified by the direct action of the environment. The most striking evidence of this sort has been furnished by Beebe, who has produced in certain birds, by means of humidity, more difference that has been accepted as sufficient for the distinction of subspecies. Leaving out all such phenomena, we still have a great series of closely allied species, with undoubtedly inherited characters, presenting the same kinds of differences as have been observed to arise by mutation, sometimes apparently as the direct result of particular stimuli. What do these phenomena mean in the practical working out of evolutionary processes?
If we know in a general way the age of particular types and the extent of their migrations, we can begin to form an idea of their practical mutability. The vertebrate paleontologist finds evidence of remarkable changes within the Tertiary period, but even he has to admit that the course of evolution is not so rapid as it might seem; that new forms suddenly appearing must surely have migrated from other regions, where they doubtless underwent a slow process of development. Central Asia, we must now think, must have been the home of various groups, and will one day yield fossils of surpassing interest. Africa, once seeming so barren paleontologically, has of late begun to yield her treasures.
The student of fossil invertebrates finds the process of change to have been, in the majority of cases, extraordinarily slow; and the paleobotanist finds it slower still. It is not that plenty of specific forms were not produced, but the generic and higher types were so little susceptible to change. A wrong impression has been produced, even among the vertebrates, by the presence in different strata of remarkable extinct groups. No doubt two or three species of elephants walking about in our mountain-parks would give a strikingly different appearance to the Colorado landscape; but the time since this actually occurred is, geologically speaking, very inconsiderable, and does not represent any great step in the process of evolution. The more I study the insects and plants of the Florissant Miocene, the more convinced I become that, speaking broadly, the extinct genera and higher groups are not the ancestors of any now living, but represent types which have failed, like the mammoth; while the real representatives of the modern biota show that there has been singularly little forward evolution in the course of perhaps a million years. Many of these are totally extinct in Colorado, but live elsewhere; thus the redwood differs little from that of California, while the wonderfully delicate and fragile Halter, belonging to a family no longer living in North America, is closely related to a living species of Persia.
Hence the experimental researches of De Vries and others, proving that mutation is a relatively common phenomenon among plants, prove perhaps too much. If change is so easy, why so little change, and that in the face of a radical change in temperature and moisture? It seems, indeed, that "elementary species" have always been produced in greater or less abundance, but by a sort of oscillation less related to the forward march of evolutionary activity than we might at first suppose. The ability to produce heritable segregates, especially in the face of adverse or strange conditions, is clearly of advantage, as giving new chances for spread or survival. Thus in the long run the tendency to break into "elementary species" would in many cases be favored by natural selection, without any necessity for each one of these, or even the majority, being directly related to a particular environment. There is no reason, apparently, why this should not continue for ages as an oscillation-process, a segregation in space rather than in time, producing thousands of species without overstepping the limits of the general group, or perhaps advancing at all in complexity. The molluscan genus Ostrea, the oysters, may be taken as an example of this; indeed, the modern oysters scarcely do justice to their Cretaceous ancestors. When it was generally held that species were created by divine fiat, it naturally appeared that he who should explain the origin of species might be given the rest without further charge. We are coming to see that there are diverse problems involved, and while the whole matter may well be locked up in the evolution of any single species, or indeed of any single cell, we begin to doubt whether we really possess the key.
Speaking philosophically, progressive or orthogenetic evolution—the existence of which no naturalist has any ground for doubting—must have a cause external to itself. All probability favors the idea that this did not operate once for all, but has continued in action throughout the ages. It may be found, perhaps, in the susceptibility of the hereditary mechanism to environmental influences of particular kinds, the nature of which remains for the present obscure. These reactions would fall under the operation of natural selection from the very beginning; thus a too susceptible organism would quickly be thrown out of gear and would perish; a too conservative one, unless adapted to practically unchanging types of life, would equally perish. There would be a certain optimum susceptibility, which would be preserved, and would differ for different groups. More than this, certain kinds of susceptibility would be favored, and being once developed might, like bad habits, become harmful through the accumulation of results, resulting in extinction. Thus rapid evolution would usually go with a high percentage of failures, and a considerable number of grotesque forms, such as we see among the vertebrates. According to this view, the initiation of any evolutionary trend, except the oscillatory movements above described, would be exceedingly slow, and quite beyond the reach of experimental methods, other than those furnished by nature in the course of ages; hence, as Osborn has indicated, the great importance of paleontological researches. At the same time, while the processes which change the fundamental character of animals and plants may be too slow to observe, it is not to be doubted that very much light may be obtained by the experimental method, if only by way of showing us what it is that has been evolved—a thing we seem not to have clearly known. If the control of orthogenetic changes is reserved, as it were, for the gods—and we, doubtless, should only make a mess of it—we may be well satisfied if we can take advantage of the oscillation processes, which experimental researches are showing to be far more extensive and much easier to control than had previously been suspected.
- ↑ Lecture delivered before the Scientific Society of the University of Colorado, January 20, 1908.