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therefore, how bright the stars should be, if we know how large and massive a star is and how much denser it is in the interior than at the surface. It is well known that stars of the same mass are all about equally bright, no matter of what size they may be. It may be shown that this is a necessary consequence of the known general laws of ionization and opacity-considerable difference in the distribution of internal density affecting very little the amount of heat which escapes from the surface. The outstanding problem is to find where the heat radiated by the stars comes from, and in what manner it is liberated inside them. The existing evidence indicates that the heat is probably produced by a slow transformation of matter into energy after the manner first suggested by Einstein. The laws governing this process can at present be investigated only by a study of the stars themselves. It can be shown that, if all the stars were of exactly the same composition, stars of the same mass would be not merely similar in brightness, but also similar in size, color and temperature. This is not the fact, and it follows that some stars must contain more than others of the "active material," which is the source of heat.

It appears that the observed facts can be accounted for by assuming that there are two kinds of active material, both of which are transformed more rapidly the higher the temperature. For the first, the rate of transformation is nearly the same at all pressures. For the second, low pressure, as well as high temperature, favors the production of heat.

It can be shown that a star of given mass will automatically adjust its diameter and internal temperature until the rate of production of heat from one or other of these active substances is just sufficient to balance the radiation from the surface. If a large number of stars of different masses should come into existence in any manner, they would adjust themselves, probably within a few millions of years, so as to exhibit just such relations between mass, brightness and color, as are actually found in star clusters and among double stars. It is therefore no longer necessary to assume that different stars in the same cluster are of very different ages, as had to be done on the earlier theories.

What the life-history of a star would be depends upon the proportion of active material in its composition. If, as seems probable on the whole, this originally forms the larger part of the star's mass, a star of large mass will start as a red giant, gradually become hotter and whiter, and finally cool down and end as a faint dwarf star. Stars of smaller mass may begin their careers as dwarf stars without ever passing through the giant stage. White dwarf stars, like the companion of Sirius, may be accounted for as cases in which one of the two kinds of active material has been completely used up; and stars of all known types appear to find a place in the scheme. These conclusions differ from those reached recently by Dr. Jeans. The writer believes that certain of Dr. Jeans's conclusions, while mathematically sound, on the assumptions which he has made, do not correspond with the conditions that are actually met with in the stars.

Barro Colorado Island as a station for the study of tropical life (illustrated): FRANK M. CHAPMAN.

On painting eclipses and lunar landscapes (illus. trated): HOWARD RUSSELL BUTLER (introduced by H. N. Russell).

Designs of a building devoted to general education in astronomy and related sciences (illustrated): HOWARD RUSSELL BUTLER (introduced by Henry Fairfield Osborn).

Orbit of a minor planet (100) Hekate: A. O. LEUSCHNER and H. THIELE.

The discovery of eclipsing stars: PROFESSOR JOEL STEBBINS.

Observations with the spectroscope reveal motions of certain stars which can be explained by their having large companions or planets. The periods of revolution of many of these attendant bodies are very short, even as small as one or two days. By choosing the proper time for light measurements, it is found that among the cases known in advance to be favorable fully one half of these double systems present eclipses as viewed from the earth. A study of the variation of the light of a star during an eclipse makes it possible to calculate the diameter of both the bright star and its dark or faint companion. As an illustration, it is noted that two stars moving in space parallel to the stars of the Big Dipper, and presumably belonging to that system, have each been found to have satellites. It is shown that each of the bright bodies is twice as heavy and gives one hundred times as much light as the sun, so that the latter would make only a mediocre planet for any star of the Big Dipper.

These observations are taken with the photo-electric cell, the same instrument that is used for transmission of pictures over telephone wires. The observer measures the light of stars by timing their effect on a delicate electrometer, attached at the eye-end of the telescope, and it is literally true that it is possible to measure and weigh a star by means of a stop-watch.

On the frequency of parallel proper motions among the stars: PROFESSOR FRANK SCHLESINGER.

Yale Observatory has recently issued a catalogue of the positions and proper motions of more than 8,000 faint stars between declinations 50° and 55° north. A discussion of these proper motions reveals many cases of stars separated by several degrees of arc whose proper motions across the sky are sensibly parallel and equal in amount. To make certain in any particular pair or group of cases that the motions are parallel in space, we should have to know the radial velocities, and these in general are not at present forthcoming. But the frequency with which the proper motions come out the same is much greater than can be due to chance and indicates that most of these cases represent true equality and parallelism of motion. If this view is correct it implies that star streaming is a common phenomenon. (To be concluded)

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SCIENCE

VOL. LXIII

MAY 14, 1926

No. 1637

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Lancaster, Pa. Garrison, N. Y. New York City: Grand Central Terminal. Annual Subscription, $6.00. Single Copies, 15 Cts. SCIENCE is the official organ of the American Association for the Advancement of Science. Information regarding membership in the Association may be secured from the office of the permanent secretary, in the Smithsonian Institution Building, Washington, D. C.

Entered as second-class matter July 18, 1923, at the Post Office at Lancaster, Pa., under the Act of March 8, 1879.

THE HISTORY OF ORGANIC

EVOLUTION1

THE meaning of evolution is probably more misunderstood than any doctrine of science. The reason is that it has been discussed very freely by those who are not informed, and in this way much misinformation has been propagated.

The general meaning of organic evolution is that the plant and animal kingdoms have developed in a continuous, orderly way, under the guidance of natural laws, just as the solar system has evolved in obedience to natural laws.

There are at least three important reasons why evolution should be regarded as a necessary part of college training.

(1) It has revolutionized modern thought. Every subject to-day is being attacked on the basis of its evolution. Not only are inorganic and organic evolution being considered, but also the evolution of language, of literature, of society, of government, of religion. In other words, it is a point of view which represents the atmosphere of modern investigation in every field.

(2) It is persistently misunderstood. From the press, the lecture platform and even the pulpit, one frequently hears or reads amazing statements in reference to organic evolution. If it were made an essential feature of student training, there would be developed a propaganda of information instead of misinformation.

(3) It has revolutionized agriculture. The practical handling of plants and animals, in the way of improving old forms and securing new ones, was made possible and definite when the laws of inheritance began to be uncovered through experimental work in evolution.

PERIODS IN THE HISTORY OF EVOLUTION There have been three distinct periods in the history of evolution, based upon the method of attack. These three methods may be spoken of in general as speculation (ancient), observation and inference (medieval) and experimentation (modern).

Lecture delivered at a joint meeting of the New York Association of Biology Teachers, the Chemistry Teachers Club of New York, the Physics Club of New York, and the Torrey Botanical Club, at the Hotel Majestic, New York City, on March 27, 1926, and arranged under the direction of the Science Committee of the Board of Education.

(1) Speculation: The idea of organic evolution is as old as our record of men's thoughts, for all the old mythologies are full of it. No modern man, therefore, is responsible for the idea, although it is a common misconception to load this responsibility upon certain distinguished modern students of evolution. For example, the name of Darwin is so conspicuous in connection with evolution that many seem to think that Darwinism and evolution are synonymous. Until 1790, however, organic evolution was a pure speculation, with no basis of scientific work. It should be emphasized that the idea of evolution has always been present in the mind of man.

During the latter part of this ancient period of speculation, certain facts began to be observed that made some thinking men conclude that evolution might be a fact, and not merely a speculation. It will be helpful to note briefly, in historical succession, the kind of facts that set these men to thinking, and that resulted in the second period in the history of evolution, when it became a science.

In classifying plants and animals, which was the initial phase of biology, men rigidly defined the different species, the thought being that the different kinds had descended in unbroken succession "from the beginning," whenever that may have been. When more extensive observations were made in the field, numerous intergrades began to be found. The species, as defined, seemed to intergrade freely. In other words, the pigeon-hole arrangement, with rigid partitions, did not express the facts. It became evident that species had been defined by man rather than by nature. Some were distinct enough, but many intergraded. This intergrading suggested that one species might come from another, the intergrades marking the trail.

The next observations suggesting that evolution might be a fact had to do with what was called the "power of adaptation," which we now call "responses." It was observed that plants and animals respond to changes in environment, often in a striking way. I have seen what were regarded as two good species changed into one another by changing from a moist habitat to a dry one, or the reverse. This ability to respond to changing conditions seemed to indicate that species are not so rigid and invariable as had been supposed.

As technique developed, and the internal structures of plants and animals became known, it often happened that rudimentary structures were found, which never developed to a functioning stage, but which occurred fully developed in related forms. For example, it was found that in the developing parrot a set of embryo teeth begins, but never matures. The inference was natural that these structures had been

functional in the ancestors, but had been abandoned by some of their descendants. In these days, it has become the habit to call these rudimentary structures "vestiges." Many such illustrations could be given. One in the human body is the vermiform appendix. It seems safe to say that we are walking museums of antiquity.

As technique developed still further, the embryology of plants and animals began to be studied in detail, the whole progress from egg to adult being observed. In very many cases, during this progress, glimpses of fleeting structures and resemblances were obtained, which had disappeared when the adult stage was reached, but which related the form to other species.

After this succession of facts, there came a revelation which convinced more men that evolution is a fact than any evidence which had preceded. The geologists had begun to uncover that wonderful succession of plants and animals from the earliest "geological periods to the present time. They saw in the oldest periods forms unlike any now existing; they saw gradual changes with each succeeding horizon; they saw a steady approach to forms like those of to-day, until by insensible gradations the present flora and fauna were ushered in. This geological record, becoming continuously more detailed in its interpretation, set men to thinking seriously.

Finally, after all this evidence was in, men began to look around them and to realize what they had been doing for centuries in domesticating animals and plants. They had been bringing them from the wild state and changing them so much by the methods of culture that in many cases the wild originals could not be recognized. Most of our cultivated plants, if found in nature associating with their wild originals, would be regarded as extremely distinct species.

In the presence of such an array of facts, is it to be wondered at that certain men began the serious, scientific study of evolution? As a result, the second period in the history of evolution was ushered in, and evolution became a science.

(2) Observation and inference: In time, this period extends from 1790 to 1900. It is characterized by the appearance of a succession of explanations of evolution. It is important to remember that the men who offered these explanations are not responsible for the idea of evolution, but merely attempted to explain the fact of evolution. They were explainers rather than authors. It is also important to realize the method used. It may be called the method of comparison and inference. Plant and animal forms were observed, and resemblances were assumed to indicate relationship through descent. It was not dem

onstration, but inference based on observation. Darhe win carried the method to the limit of its possibilities, observing not a small range of forms, but observing through several years a world-wide range of forms, fon in connection with the famous voyage of the Beagle. His caution is also indicated by the fact that his observations were under consideration for some twenty years before his conclusions were published.

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This second period in the history of evolution, which we may call the medieval period, is marked by the appearance of several explanations. I shall mention only the three most conspicuous ones, and there is no need to define these in detail.

The explanation which ushered in the period was proposed simultaneously and independently in 1790 by Goethe, of Germany, St. Hilaire, of France, and Erasmus Darwin, of England. Observations of repsponses to changed environment led them to the conclusion that environment is the direct cause of change, actually molding forms. This evolutionary factor, therefore, is entirely external to animal or plant. It was a natural first explanation, but of course it was too superficial, and environment as a direct cause of evolution soon passed into the historical background. It deserves mention only because it was the first attempt at an explanation. In 1801 Lamarck, in a series of lectures, announced his explanation, calling it the theory of "appetency." This was really the first explanation with a body of doctrine, and hence Lamarck has often been called the "founder of organic evolution." The term "appetency," however, has been abandoned, and its real meaning expressed by the phrase "the effect of use and disuse." With Lamarck, environment is not the direct cause of the change, according to the earlier explanations, but the occasion for the change. The cause is the striving, the effort to do something that had become necessary. Thus organs would become developed as a consequence of some change in environment calling them into use; and, conversely, organs would gradually become aborted as a consequence of some change in environment that eliminated their use. planation rests absolutely upon the inheritance of acquired characters, meaning characters not inherited by the possessor, but acquired during the life of the individual.

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In 1858 the epoch-making explanation of Darwin was announced, an explanation which was dominant for about fifty years. It is too familiar to need explanation. In brief, it claims that nature selects among variations, that the method of selection is competition, that the result is the destruction of the relatively unfit, or as Spencer puts it, "the survival of the fittest." In brief, the theory is really an explanation of what is called adaptation.

As facts multiplied, the current explanations of evolution were found to be inadequate to explain some of them. This led to a general misunderstanding of the situation by the uninformed public. For example, more intensive study developed the fact that Darwin's explanation does not always explain. His name is so identified with evolution in public thought that this criticism of the universal application of his conclusions by certain scientific men was taken to mean that the theory of evolution was being abandoned. The real situation is that every proposed explanation may prove inadequate, and yet the fact of evolution remains to be explained.

All the explanations offered are partial explanations, which simply means that no one of them applies to all the facts. We need them all and more besides. So far from being abandoned, evolution is the basis of all biological work to-day.

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The method of comparison and inference continued until the beginning of the present century. came a new epoch in the history of evolution.

(3) Experimentation: This may be called the modern period, in contrast with the medieval and ancient periods. It was ushered in by the work of DeVries, who introduced the experimental study of evolution, and announced his explanation of evolution by means of mutation. The problem was to discover whether one species actually produces another one. It had been inferred that it does, but inference is not demonstration. By means of carefully controlled pedigree cultures, DeVries discovered a plant in the actual performance of producing occasionally a new form among its numerous progeny. This form bred true and preserved its distinctive characters; in other words, it was a new species or at least a different species from its parent. Many such species have now been observed originating in this way, both in plants and animals. That one species can produce another one is no longer inferred, but demonstrated, and demonstrated repeatedly. There is no longer any doubt, therefore, that evolution is a fact. It is quite a different question whether the proposed explanations are adequate.

When inferences were the only results, in the medieval period of evolution, it was natural to extend inference to the evolution of the plant and animal kingdoms, and this involved the origin of man. In these days there is no such attempt, for experimental demonstration of the evolution of the whole series of organic forms, culminating in man, is clearly impossible. Biologists, therefore, are no longer concerned with the whole story of evolution, but only in discovering experimentally how one species may produce another one.

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lished, but the whole story of evolution must remain an inference.

PRESENT STATUS OF EVOLUTION

Only a very general statement can be made of the present status of evolution, since a full statement would involve an extensive discussion. The experimental study of evolution has led to the development of the field of genetics ("heredity"), a subject which has grown with remarkable rapidity. It is genetics which must uncover the machinery of evolution, which of course is fundamentally a matter of inheritance. The facts thus far uncovered indicate complexities which were not realized before, but which should have been anticipated, for inheritance, with its resulting evolution, represents the most complex biological situation imaginable.

The present status of evolution as a body of doctrine may be said to be in a state of flux, out of which the truth will emerge eventually. Any meeting of biologists at which evolution is discussed discloses considerable diversity of opinion, not as to the fact of evolution, but as to some attempt to explain the process.

It is evident, of course, that whatever produces variation furnishes a basis for evolution. But what produces variation? Environment is one factor; sex is another factor, especially when strains are crossed; and other factors might be cited. Any factor claimed to induce variation must stand the test of genetics. Variations, however produced, are of two general kinds, as indicated by behavior, namely, the so-called continuous variation of Darwin's explanation, and the so-called discontinuous variation of DeVries's explanation. The differences of opinion have to do with the method of variation production, that is, variation that may result in a new species.

After such variation is secured, there is no question as to the function of selection. It is merely a statement of fact to say that some variations persist and some are eliminated. It is a very different matter to claim that only the "fit" persist. In some way the selection is made, and the selection factors may be quite variable. In general, it may be said. that there is no serious difference of opinion that evolution is based on variation and subsequent selection. It is only a matter of detail to determine the exact factors.

There is a much more serious problem of evolution, however, which is still baffling. The variations observed, which result in new species, as tested by genetics, and for which the cytological machinery has been observed, produce species either laterally or retrogressively; that is, species of the same rank or of declining rank. There is as yet no adequate ex

planation of progressive evolution, the advance from one great group to another of higher rank. Progressive evolution is a very evident fact, as shown by many an impressive series disclosed by the geological records. The theory of "orthogenesis" is often cited as an attempt to explain progressive evolution. Orthogenesis is not an explanation, however, but a name for progressive evolution. The fact remains to be explained. The multiplication of species is within the reach of experimental study as to causes and methods, and the results are leading to conclusions that may vary with the investigator, but which will be checked up by further investigation. The progressive advance of species, however, is still within the region of inference. It is something like the difference between the tracks in a switch-yard and the main line. We have succeeded in investigating the switching, but the through trains are baffling.

PRACTICAL RESULTS

I wish now to call attention to the practical results that the study of evolution has made possible. The experimental study of evolution, leading to the development of the science of genetics, resulting in increasing knowledge of the laws of inheritance, has led to practical results which the public in general do not appreciate. I shall select only one illustration from very many, but it will serve to indicate the sort of service the study of evolution has rendered in a practical way, in addition to its service in the advancement of knowledge. I have selected the revolution in agriculture. It seems a far cry from speculations concerning evolution to a revolution in agriculture, but the continuity is unbroken. Speculation led to observation; observation led to experimentation; experimentation resulted in discovering laws of inheritance; and the application of these laws has enabled us to handle plants and animals in a way that was never dreamed of before. It is a good illustration of the fact that there is no sharp dividing line between what are called pure science and applied science, for pure science may prove immensely practical.

A very brief statement will illustrate the agricultural results in the application of our knowledge of inheritance. It had become evident, for example, that for various reasons the ratio of increase in population was much greater than the ratio of increase in food production. The statement was made that during the ten years preceding the great war our population had increased 20 per cent. and our food production about 1 per cent. It was certainly an alarming outlook. Under these circumstances, plant crops began to be studied from the standpoint of genetics, and plant breeding became a science.

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