Journal of Heredity 8:
THE MODERN IDEA OF EVOLUTION
Darwin's Work Corrected and Amplified, But Its Essential Parts Not Supplanted—
Cause of Variations Still a Mystery But Manner of Their Inheritance Made Clear
LOOK at the surface of a pond ruffled by the wind, and you see that it is constantly changing. ts aspect is different in each two successive seconds, and yet after an hour of this perpetual change, the pond looks just as it did when you began to observe it. Ceaseless change here produces nothing new; when the wind dies down in the evening the pond is no different than it was in the calm before dawn.
But ever since men began to leave their thoughts on record, they have made note of the fact that this is not the most usual sort of change. Thinkers have recognized that everything is changing, and that when a living thing changes, it usually becomes something different from what it was before. They have watched the progressive change of a seed to a full-grown plant, of an egg to an adult bird; and men with a historical viewpoint have noted that similar changes occur in institutions and in nations, a small beginning leading through a long series of changes to a new condition, where differentiation took the place of the original simplicity. Greek and Hindu speculators surmised that this condition of progressive change applied to species of animals and plants, one following another, so that from simple beginnings complex creatures finally developed.
This idea of progressive change, of development and differentiation, is the idea of evolution. There is nothing new about the idea—it is not at all a product of modern science. As a philosophical speculation it has existed certainly for several thousands of years, and many attempts have been made to convert it from a pure speculation into a demonstrated theory.
During the past century, the attempt to find an explanation of the way in which evolution might take place has been more persistent than ever before. Four main lines of speculation may be distinguished.
FOUR HISTORIC EXPLANATIONS
1. It has been suggested that the environment acts directly on living creatures and causes changes in them. This view was elaborated about the beginning of the last century by Geoffroy St. Hilaire. He assumed that as the surroundings of a plant or animal change, the plant or animal itself must of necessity respond by a change. He did not assume that the response to the new environment was always a favorable one or, as we say, an adaptation. If it was unfavorable, the individual or the race died out. If it was favorable, the individual or race was able to meet the requirements of its changed surroundings, and survived.
St. Hilaire was unable to secure general acceptance of his theory, because he offered no adequate proof that things ever happened as he described them; yet his conception of evolution contains elements that form the background of our thinking today, and within the last few years his explanation has been revived in a mystical form by the French philosopher Bergson, who has secured for it a certain popularity among laymen—not among biologists.
2. The second of the four great historical explanations appeals to a change not immediately connected with the outer world, but to one within the organism itself. It suggests that any organ or structure of an individual that is much used will increase in size and strength, and this increase will be transmitted to the individual's descendants. Similarly, it is supposed that disuse will bring about a decrease in size or strength which will likewise be inherited. This theory of the inheritance of the effects of use or disuse—of the inheritance of acquired characters, to use the customary title—is associated principally with the name of the French zoologist, Lamarck. It was accepted by Darwin, who made much use of it. It held until a generation ago, when August Weismann discredited it by an appeal to common sense. Today the theory has few followers amongst trained investigators, but it still has a popular following that is widespread and vociferous.
|THE GREATEST CONTRIBUTOR TO GENETICS|
|Around any decaying fruit may be found little red-eyed flies, about one-eighth of an inch long, which are known to science under the name of Drosophila. They are so insignificant that the housewife regards them as hardly even a pest; but to a geneticist they are perhaps the most important insect in the world, for their study has thrown more light on heredity than has that of any other one animal. They are, in fact, practically an ideal subject for study, since they stand confinement well, can be kept indefinitely in a milk bottle with a little overripe banana, and in ten days a single pair of parents will produce a generation of 200 or 300 young. Further, they have a number of well-defined characters that can be followed in heredity, and the number of chromosomes is conveniently small (four). Although geneticists have been searching for ten years to find another equally satisfactory insect, they have failed. Photomicrograph by John Howard Paine. (Fig. 7.)|
3. There is a third explanation of evolution which has taken protean forms. At one extreme-it is little more than a mystic sentiment to the effect that evolution is the result of an inner driving force. The earliest name of prominence associated with it is that of Nageli; recently the geneticist, William Bateson, has put forward a theory of evolution of somewhat similar nature. The numerous theories of this type may be collected under the title of "The theory of the unfolding principle."
4. Darwin and Wallace put forward the last of the great historical speculations about evolution in the well-known theory of natural selection, and the former brought together so much evidence as to win almost universal support from men of science. He appealed to chance variations as supplying evolution with the material on which natural selection works. If we accept for the moment this statement as the cardinal doctrine of natural selection it appears that evolution is due (1) not to an orderly response of the organism to its environment, (2) not in the main to the activities of the animal through the use or disuse of its parts, (3) not to any innate principle of living material itself and (4) above all not to purpose either from within or without. Darwin assumed that small variations are constantly appearing in a species, and that individuals with favorable variations survive and transmit them to their descendants, those with unfavorable variations perish.
Darwin's original contribution was not to demonstrate that evolution existed. That fact had long been recognized in speculation and later received overwhelming proof from comparative anatomy, embryology, paleontology and breeding. All of this evidence showed that living species were related to each other, that they had so much in common as to force the assumption that they came from common ancestors; and paleontology was even able to bring forward some of these common ancestors.
The question was not to show that divergent forms had a common origin, but to explain how they had come to diverge, and Darwin's explanation of the action of natural selection on inheritable variations was generally accepted as accurate. But his attempts to explain (1) the origin of these variations and (2) the manner in which they were inherited met with little success, and as long as these two problems were unsolved, the evolutionist was on uncertain ground.
RECENT PROGRESS GREAT
Biologists, therefore, made a widespread attack on these two problems, and it is the belief of many that within the last decade (2) has been explained and (1) has been considerably elucidated, although not yet solved.
|1A Critique of the Theory of Evolution, by Thomas Hunt Morgan, professor of experimental zoology in Columbia University. Lectures delivered at Princeton University in February and March, 1916. Pp. 197, price $1.50. Princeton University Press. The book is lavishly illustrated with 95 figures, but many of them are hackneyed or inartistic. Someone could perform a great service to evolutionists by getting together a new set of illustrations to take the place of those that have been doing duty so long.|
This is the viewpoint taken by Prof. Thomas Hunt Morgan, of Columbia University, who has brought together in book form1 four lectures in which he critically examines the theory of evolution. In addition to its intrinsic merits, the book has added interest because, in the first place, Dr. Morgan was one of the first zoologists successfully to challenge the adequacy of Darwin's explanations, and still more, in the second place, because his own work has been very largely responsible for clearing up problem (2) above-mentioned, the problem of how variations are inherited. The book is simply written, and furnishes the first authoritative account of Morgan's work which is available to anyone but the specialist, Morgan's previous works having been too technical for comprehension by the reader who had no previous knowledge of genetics.
The preceding paragraphs have summed up Morgan's statement of the case. We have evolution as a theory, many attempts to demonstrate it, and finally general agreement that Darwin had demonstrated it successfully. Then we see certain fundamental parts of Darwin's proof challenged and, by many critics, thrown out of court. The questions of (1) how variations originated and(2) how they were inherited had to be attacked again, and they were fundamental to the problem. Dr. Morgan's book is mainly devoted to an explanation of his views on these two points.
We can dispose of the first of these points quickly, for the problem it presents has not yet been solved. Darwin borrowed Lamarck's view, that variations originated through the activities of the animal, and whatever could not be explained by this he admitted to be at that time unexplainable. We have now thrown out the Lamarckian view, and if we have not been able really to explain how variations arise, we have at least, Dr. Morgan thinks, a truer view of what happens. His discussion of this subject is perhaps the least satisfactory part of the book, but his position is clear enough, being based on the developments of Mendelian heredity.
|THE CARRIERS OF HEREDITY|
|Many different lines of study have made it seem probable that much, at least, of the heredity of an animal or plant is carried in the nucleus of the germ-cell and that in this nucleus it is further located in little rods or threads which can be easily stained so as to become visible, and which have the name of chromosomes. In the above illustration four different views of the nucleus of the germ-cell of an earthworm are shown, with the chromosomes in different stages: in section 19 each chromosome is doubled up like a hairpin. Study of the fruit-fly Drosophila has made it seem probable not only that the hypothetical factors of heredity are located in the chromosomes, but that each factor has a perfectly definite location in its chromosome; and Dr. Morgan and his associates have worked out an ingenious means of measuring the distance from either end, at which the factors lie. Photomicrograph from Foot and Strobell, Archiv. f. Zellforschung V, p1. xii. (Fig. 8.)|
NATURE OF VARIATIONS
Darwin thought of the individual as a unit, which was undergoing variations in all parts. As a result of experimental breeding, Dr. Morgan says, we must now abandon this view. The animal itself is not the unit which varies; the germ-plasm of the animal is what varies, and the animal (or plant) is merely the product of (hypothetical) germinal factors. The importance of this change in viewpoint may not be apparent at first sight, but in practice it is found to be weighty, for it substitutes precision and clarity of thought for a wholly mystical idea, and enables us to breed understandingly.
Morgan quotes many instances of variation in the fruit fly Drosophila, on which his work for the last ten years has been mostly based. It is a variable creature; if we catch a bottle full of the same species, we will find that in details they offer numerous differences, and if we go on to breed them we will find that many of these differences are passed on to their progeny. These differences are further found to be inherited in what is known as Mendelian fashion, with which the reader is doubtless familiar.
The point of interest is that every once in a while a fly is born with some character different from the corresponding character of his parents. The parents had normal wings, for example, the offspring comes into the world with extra long wings, and this extra length is found to be inherited in a very definite way.
It is further discovered as the result of long, careful, and widespread observation (Morgan and his associates and students have bred more than half a million flies altogether) that with this change in wing-length go certain other changes in the fly. A change, a variation, seems never to affect one part of the fly alone; it has an indefinitely large number of effects in various parts of the body (and, must we not suppose, on the "mind," too?).
To be strictly accurate, then, we should not say that a certain variation affects length of wing, but that its chief effect is to lengthen the wing. "For example, a mutant stock called rudimentary wings has as its principal characteristic very short wings. But the factor for rudimentary wings also produces other effects as well. The females are almost completely sterile, while the males are fertile. The viability of the stocks is poor. When flies with rudimentary wings are put into competition with wild flies relatively few of the rudimentary flies come through, especially if the culture is crowded. The hind legs are also shortened. All of these effects are the results of a single factor-difference.
BY-PRODUCTS OF VARIATION
"One may venture to guess that some of the specific and varietal differences that are characteristic of wild types and which at the same time appear to have no survival value, are only by-products of factors whose most important effect is on another part of the organism where their influence is of vital importance."
"I am inclined to think that an overstatement to the effect that each factor may affect the entire body, is less likely to do harm than to state that each factor affects only a particular character. The reckless use of the phrase 'unit character' has done much to mislead the uninitiated as to the effects that a single change in the germ-plasm may produce on the organism. Fortunately, the expression 'unit character' is being less used by those students of genetics who are more careful in regard to the implications of their terminology."
Now to sum up the new view of the problem of the nature and origin of variations, which Darwin failed to solve. As to the actual cause of these changes in the germ-plasm, we know no more than he did; we suppose them to be chemical reactions. But as to their effects, as to what variation actually means, we have learned a great deal. We have found out that the germinal differences of an individual are inherited separately from each other; that every change in the germ-plasm—i.e., every variation—affects not one but a large number of characters; and conversely, of course, that every visible character is the result of the concurrent action of a large number of factor-differences or variations. It has likewise been demonstrated in many cases that these variations are inherited in a perfectly definite and predictable way, in accordance with the laws of Mendel.
If the problem of the origin of variations has not actually been solved, we at least have reached a much more exact comprehension of it. We have learned that the animal's own activities can not be invoked to account for variations, and that the environment's direct action cannot explain them. We have seen them to be due to changes in the structure of the germ-cell. It is not certain that we can produce experimentally a single inherited variation—our knowledge is, therefore, much lacking; but the investigators can certainly "report progress," even though it be mainly of a negative kind.
If we turn to the second of the great problems, namely, the way in which variations are transmitted, the nature of the mechanism of heredity, we get a more favorable report. Here are Dr. Morgan's own words:
"THE PROBLEM OF HEREDITY
"I have passed in
review along series of researches as to the nature of the hereditary material.
We have, in consequence of this work, arrived within sight of a result that a
few years ago seemed far beyond our reach. The mechanism of heredity has, I
think, been discovered—discovered not by a flash of intuition but
as the result of patient and careful study of the evidence itself.
"With the discovery of this mechanism I venture the opinion that the problem of heredity has been solved. We know how the factors carried by the parent are sorted out to the germ cells. The explanation does not pretend to state how factors arise or how they influence the development of the embryo. But these have never been an integral part of the doctrine of heredity. The problems which they present must be worked out in their own field. So, I repeat, the mechanism of the chromosomes offers a satisfactory solution of the traditional problem of heredity."
|2See "Mendelism up-to-date," a Review of "The Mechanism of Mendelian Heredity," by Morgan, Sturtevant, Muller and Bridges. JOURNAL OF HEREDITY. Vol. VII, pp. 17-23, January, 1916.|
It is not necessary here to describe this mechanism in the germ-cells, for an account of Dr. Morgan's work on it was presented in the JOURNAL OF HEREDITY only a few months ago.2 The germ-cells contain little rods of easily stained material to which the name of chromosomes has been given, and changes that occur in the composition of these rods seem to result in the end-effects which are seen in characters of the animal or plant.
In his last chapter, Dr. Morgan returns again to the doctrine of natural selection, Darwin's principal contribution to the problem of finding how evolution takes place. "In his great book on the Origin of Species Darwin tried to do two things: first, to show that the evidence bearing on evolution makes that explanation (i.e., evolution) probable. No such great body of evidence had ever been brought together before, and it wrought, as we all know, a revolution in our modes of thinking.
"Darwin, also, set himself the task of showing how evolution might have taken place. He pointed to the influence of the environment, to the effects of use and disuse, and to natural selection. It is to the last theory that his name is especially attached. He appealed to a fact familiar to every one, that no two individuals are identical and that some of the differences that they show are inherited. He argued that those individuals that are best suited to their environment are the most probable ones to survive and leave offspring. In consequence their descendants should in time replace through competition the less well-adapted individuals of the species. This is the process Darwin called natural selection and Spencer, the survival of the fittest."
OBJECTIONS TO DARWIN'S VIEWS
The objections to it,
Dr. Morgan says, are (1) that selection can not actually produce anything new
and (2) that selection after a while loses its effect. These objections open up
controversies which cannot here be pursued. Adopting for the moment Dr.
Morgan's views, "The question still remains: Does selection play any role
in evolution, and, if so, in what sense? Does the elimination of the unfit
influence the course of evolution, except in the negative sense of leaving more
room for the fit? There is something further to be said in this connection,
although opinions may differ as to whether the following interpretation of the
term 'natural selection' is the only possible one.
"If through a mutation a character appears that is neither advantageous or disadvantageous, but indifferent, the chance that it may become established in a race is extremely small, although by good luck such a thing may occur rarely. It makes no difference whether the character in question is a dominant or a recessive one, the chance of its becoming established is exactly the same. If through a mutation a character appears that has an injurious effect, however slight this may be, it has practically no chance of becoming established.
"If through a mutation a character appears that has a beneficial influence on the individual, the chance that the individual will survive is increased, not only for itself, but for all of its descendants that come to inherit this character. It is this increase in the number of individuals, possessing a particular character, that might have an influence on the course of evolution."
If the word variation be substituted for the word mutation in the preceding three paragraphs, the statement would appear to be a good outline of Darwin's own position. The question of how much Morgan's view differs from Darwin's is, then, a question of the difference between Darwin's variations and Morgan's mutations. To the reviewer, the difference appears one rather of words than of facts. The mutations in Drosophila, which Morgan describes at some length, are many of them exactly the kind of changes which Darwin described as variations.
A DIFFERENCE OF WORDS
If this is a fair statement, then Darwin's work has not been supplanted to anything like the extent that is sometimes supposed. A decade ago, when the mutationists were young and enthusiastic, they gave the impression that they were about to make a great change in the status of natural selection. Darwin's variations were challenged as not being inheritable (as a fact, many of them were not) and mutations were put forward as the real basis of evolution. After ten years of study, it appears that mutation and variation mean practically the same thing. A lot of mere fluctuations have been thrown out, but the concept of mutations as described by Dr. Morgan, while more definite, is not radically different from that of variations which Darwin recognized. That the two parties should now stand so nearly on common ground (whether they recognize it or not) argues well for the validity of the ideas they hold.
Next, does selection of a certain kind of variations lead to the probable appearance of further variations in the same direction? Dr. Morgan thinks not. The only role of selection is to multiply the numbers of some favorable variation, thus giving it a chance to become established and crowd out the older forms. His conclusion follows:
shows clearly that the characters of wild animals and plants, as well as those
of domesticated races, are inherited both in the wild and in the domesticated
forms according to Mendel's Law.
"The causes of the mutations that give rise to new characters we do not know, although we have no reason to suppose that they are due to other than natural processes.
"Evolution has taken place by the incorporation into the race of those mutations that are beneficial to the life and reproduction of the organism. Natural selection as here defined means both the increase in the number of individuals that results after a beneficial mutation has occurred (owing to the ability of living matter to propagate), and, also, that this preponderance of certain kinds of individuals in a population makes some further results more probable than others. More than this, natural selection cannot mean, if factors are fixed and are not changed by selection."
Most Mendelians would accept Dr. Morgan's statement as their own. Parts of it would be seriously questioned by other geneticists and by many zoologists and botanists and all biometricians. But these parts are minor ones, and the essential differences between these various workers appear to be somewhat exaggerated by the use of different names for the same thing.
DARWIN NOT DISCREDITED
If so, it follows that natural selection stands in almost the position where Darwin left it. It is still the only acceptable account of how adaptation takes place, unless one is content to accept a mystical explanation. The past half century has seen the elimination of many false hypotheses; it has brought a clearer idea of the nature of variations and a great increase in the knowledge of how they are inherited; but back of all this is Darwin's principal work—the hypothesis of natural selection—which is substantially confirmed. The reviewer is unable to avoid the conclusion that Dr. Morgan exaggerates the differences between the Mendelian view and the original Darwinian view. Fifteen years of Mendelism have brought much increase of knowledge, and this knowledge has made many ideas of evolution clearer; but it does not seem materially to have changed the theory of natural selection which Darwin built up. As far as the evidence goes, we must still look on evolution as due to the action of natural selection on variations (or mutations), although we have a much clearer idea of the nature of these, and the mode of their inheritance, than was possible to the last generation.
CybeRose note: Darwin (Origin of Species) would not have approved of this "correction" to his theory because he expressively rejected the "mutations" Morgan so admired.
"If selection consisted merely in separating some very distinct variety and breeding from it, the principle would be so obvious as hardly to be worth notice; but its importance consists in the great effect produced by the accumulation in one direction, during successive generations, of differences absolutely inappreciable by an uneducated eye—differences which I for one have vainly attempted to appreciate. Not one man in a thousand has accuracy of eye and judgment sufficient to become an eminent breeder. If gifted with these qualities, and he studies his subject for years, and devotes his lifetime to it with indomitable perseverance, he will succeed, and may make great improvements; if he wants any of these qualities, he will assuredly fail. Few would readily believe in the natural capacity and years of practice requisite to become even a skilful pigeon-fancier."
These changes are clearly not the same as those observed by Morgan.
Morgan and his students may have raised more than half a million flies, but they were not trying to raise better fruit flies that could out-compete their wild cousins.
The notion that Morgan "corrected" Darwin's theory accounts for the fact that some uninformed people today believe that it was Darwin himself who claimed that gene mutations were responsible for evolutionary change. Apparently such people are too busy to read Darwin for themselves.