Creation by Evolution/Can We See Evolution Occurring?

by Herbert Spencer Jennings

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CAN WE SEE EVOLUTION OCCURRING?

By Herbert Spencer Jennings

Professor of Zoölogy, The Johns Hopkins University

The doctrine of organic evolution is the doctrine that animals and plants are slowly transforming, producing new kinds; that they have done this in the past and are continuing to do it now. It does not deal with something transcendental, something metaphysical; it deals with processes as real as the running of a stream or the growth of a tree. Organic evolution, then, is a physiological process, like the digestion of food; it is something that is occurring at all times, including the present. The doctrine of organic evolution means simply that if you lived long enough you would see organisms begin as simple creatures, change shape and structure as a growing plant does, become diverse, transform repeatedly, until from one or a few types many would be produced. You would get dissolving views of amoeba transforming to creatures having more definite structures and greater complexity; of Hipparion becoming a horse; of an ape-like creature becoming a man.

But no one can live long enough to see all that. The process is too slow. Within the life of a man, or of many successive men, very little alteration can be expected. Yet men have detected and measured other slow things. The earth’s pole swings about in a small circle with a movement so slow that it requires 25,000 years to go once around the circle; yet men have discovered this fact and have measured the present rate of the motion. The fixed stars are not in fact fixed in their relation to one another; slight changes of position occur, some of them requiring centuries for detection; yet men have detected them. Certain radioactive substances disintegrate so slowly that it requires millions of years for a given portion to transform, yet the changes have been detected and their rate has been measured. If we can detect these things why should we not be able to detect—to catch in progress—the changes that we call evolution? We cannot directly see the growth of a tree, but by taking photographs at intervals and running them through a moving picture machine we can see the tree grow, and we can determine how its growth occurs. Ought we not to be able to get some sort of a moving picture of evolutionary change?

The task is bound to be difficult. The process of evolution is complex. Evolutionary changes move in many different directions. Some organisms degenerate; others grow more complex and become adapted to more varied conditions; still others change hardly at all. The process cannot be uniform; it produces diversity, not simplicity. What sort of changes should we expect to find if we could watch certain organisms closely enough to see the evolutionary changes that occur in them during a human lifetime?

We should have to study some organism that produces many generations while man passes through but one. Fortunately, there are many such organisms—creatures that produce a new generation every day or every few days. We should have to begin with a single individual and follow its offspring for many generations, obtaining great numbers of descendants. According to the theory of evolution, slight changes would occur as the generations pass. These changes would not all be purely transient, disappearing with the next generation; some of them would be hereditary; they would persist in later generations. The descendants of the original single individual would become diverse—hereditarily diverse. From a single individual, from a single race, we should thus, after the passage of many generations, get many races that would be hereditarily diverse.

In a human lifetime or in many human lifetimes we could not expect these changes to be great. Geological time is enormously long and evolution is prodigiously slow. The doctrine of evolution would therefore not lead us to expect to see widely diverse creatures produced. The popular demand that we should see a cat, or the offspring of a cat, transformed into a dog, or an amoeba into a vertebrate, is not in accord with the doctrine of evolution. We cannot expect in a lifetime to see new “species” produced. All that the doctrine of evolution leads us to expect is that there should appear slight hereditary changes, so that from a single race there are produced a number of hereditarily diverse races, differing slightly.

Do we find this? Studies of this sort have been made of a number of organisms. What was found in such a study made by the present writer may be set forth as a type.

It is common to suggest that amoeba or some amoeba-like creature is the original stock from which animals descended; “from amoeba to man” is a common phrase. It is of interest to examine amoebas from this point of view. Are amoebas still transforming, producing other kinds of animals? Some of the amoebas are naked and formless, so that the detection of any slight hereditary changes would be almost impossible. Others have shells of definite form and structure, furnishing excellent opportunity for the detection of hereditary alterations. These shelled amoebas, though they closely resemble

Fig. 1.—Microscopic views of a shelled amœba.

A medium-sized amœba is over a million million times smaller than the smallest mammal.

Shells of two diverse races of Difflugia corona, showing characteristics that become hereditarily altered as generations pass. The individual shown at a and b (a side view and a view looking at the mouth) bears about seven long spines and has sixteen teeth around the mouth; the individual shown at c and d has four small spines and fourteen teeth.

the naked ones, are designated by other names. One called Diffiugia corona (Fig. 1) was selected for observation and breeding. It is a miscroscopic creature about 1–150th of an inch in diameter.

These creatures multiply for long periods without any sexual process; that is, each individual divides into halves, and each half then develops into a complete cell, which is later in turn subjected to the same dividing process. Any individual is therefore the offspring of but a single parent; not of two parents, as in the higher animals. The method of reproduction in Difflugia is shown in Figure 2. A new generation is produced about every two to four days, so that in the course of a year or two many generations may be followed through thousands of descendants produced from one individual.

Do these thousands of descendants all remain hereditarily alike? Or do they gradually and slowly diverge, becoming hereditarily different, as the doctrine of evolution sets forth?

This was studied by allowing a single individual to reproduce for many generations, until it had produced thousands of offspring. In the early generations of such an experiment, hereditary changes cannot be detected. The offspring often differ from the parents in certain respects, but most of these differences appear not to be inherited. The next generation shows similar differences, but as the generations increase in number we find that certain diversities accumulate and become hereditary. In some descendants the spines become longer; in others they remain shorter. In some the bodies are larger; in others they are smaller. Different combinations of size of bodies and of length of spine appear. These differences are inherited. In time from the original single individual a number of diverse stocks have been formed (Fig. 3, B to F.). These five sets are like different branches of a tree, all coming from one trunk. They are separated by many generations from the original parent A. During the passage of these generations the different branches have become permanently diverse. Each differs from the other hereditarily. Even when all are living

Fig. 2.—Method of reproduction of Difflugia (after Verworn).

The parent A consists of a mass of protoplasm, covered by a rounded shell made of sand grains cemented together. This shell has an enlarged opening (below, in the figures). During its life the parent creeps about at the bottom of pools. It takes up many sand grains, which it stores within its body. At reproduction the protoplasm of the parent swells and projects from the mouth of the shell A. This projecting part enlarges and takes a form similar to that of its parent (at B and C). The nucleus of the parent divides and one-half of it passes into the projecting portion. The sand grains within the parent body also pass out into the projecting mass, come to its surface, and spread over it (C and D). They are embedded in a fluid secretion which now turns hard, forming a shell like that of the parent. The two shells are in contact at their mouths. Now the mass of protoplasm divides into two individuals, which separate, one retaining the old shell (above in the figures); the other having the new shell. Later, each individual repeats this process, producing another generation.

under the same conditions the stocks remain diverse for generation after generation. From the original single stock several hereditarily diverse stocks have been produced. Each set or race included a large number of individuals, all showing the characteristics illustrated by their representatives in the figure. A single stock, derived by fission from a single parent, has gradually diversified itself into many stocks that are hereditarily different.

What the doctrine of evolution asserts is therefore true for Difflugia. It does gradually transform and produce new races. If this is what evolution means, we have here seen evolution occurring.

A number of other lower organisms have been studied in a similar way, and with similar results. They do not remain entirely constant. Although the process is excessively slow, they gradually transform into hereditarily diverse races, in accordance with the doctrine of evolution.

To observe such changes in higher animals and plants is much more difficult. Each generation requires a longer time; in a human life few can be observed. But a greater difficulty lies in the fact that most of the higher organisms reproduce from two parents. The two parents always differ in their hereditary constitution, so that the offspring are usually a combination of two hereditarily diverse stocks. In forming that combination, each parent loses half of its genes—that is, half of the thousand different chemicals on which depend the way it develops and its later characteristics. The remaining halves from the two parents then unite to form a new combination of genes, from which the offspring develops. For every single offspring the process is repeated, but in each case it is a different set of genes that is lost from each parent, a different set that remains. Consequently through the union of the two remaining halves there is in every case a new and diverse combination of the genes produced; so that every one of the offspring of a pair of parents differs in its hereditary constitution from every other one;

A, Original parent from which B, C, D, E, F are derived.

B, Individuals small, with small spines.

C, Bodies somewhat smaller, with larger spines.

D, Spines still larger; bodies about the same size as C.

E, Animals larger than A, B, or C and spines larger than in C.

F, Bodies larger than others, but spines small.

Fig. 3.—Five different races of Difflugia corona derived from a single stock.

as well as from both of the parents.^[1] These differences show themselves in the characteristics of the developed individuals, in thousands of diverse ways, some very marked, some extremely inconspicuous. It becomes therefore extremely difficult to distinguish differences produced in this way—by recombinations of genes in biparental reproduction—from differences that are steps in evolution. In most higher organisms this is indeed at the present time impossible.

Yet in certain higher organisms these difficulties have been overcome. By study, continued for years, the hereditary constitutions of the parents are thoroughly learned, so that the results of their combination are known. In such organisms the hereditary constitution does change at times, irrespective of the recombinations due to the union of two parents. The changes are infrequent. Yet in such an animal as the fruit-fly (Drosophila), studied by Morgan and his disciples, so great has been the number of individuals and of generations minutely studied that literally hundreds of different alterations in hereditary characters have been observed. Drosophila has given rise to hundreds of new stocks, which differ permanently from the original one. Some of the changes are strongly marked, as when red-eyed animals suddenly produce a white-eyed race, or when long-winged creatures suddenly produce a race that is permanently without wings. These very marked changes were naturally the first ones observed, so that for a long time it was believed that all evolutionary changes were large leaps, saltations. But since acquaintance with the animals has become more minute it has been discovered that extremely slight, almost imperceptible, changes in hereditary characters are much more common than large ones. Dozens of different faint gradations in the color of the eye have appeared. Physiological changes so slight that they can be perceived only after long experimental study have been noted in great numbers. Every feature of the animal has thus become modified in many different ways. Hundreds of diverse races of Drosophila have taken origin from the original one; and many of these are more diverse than what have been called different species in this genus.

We do not yet understand the causes of these changes; we do not know how they are produced. An immense deal remains to be learned about them. But our ignorance must not be allowed to obscure the great, the essential fact that appears in these attempts to see evolution in progress—the fact of actual change. Remember that there are two opposite doctrines. One holds that the constitution of organisms is permanent; that they were created as they are and do not change. The other, the doctrine of evolution, holds that the hereditary constitution slowly changes as generations pass; that a single race differentiates in the course of time into diverse ones; that from one stock many are produced. The critical observations that have been made on these minute living organisms through the passage of generations substantiates this theory; they do change and differentiate into diverse races as generations pass. The facts observed are what the doctrine of evolution demands, not what the opposed theory demands.

REFERENCES

Note: A full, illustrated account of the studies of evolutionary change in Difflugia is given in an article entitled “Heredity, Variation, and the Results of Selection in the Uniparental Reproduction of Difflugia corona,” by H. S. Jennings, published in Genetics, vol. 1, 1916, pp. 407–534. A comparative account of these experiments and similar ones on other organisms is given in the author's book “Life and Death, Heredity and Evolution in Unicellular Organisms” (R. Badger, Boston, 1920) . An account of racial changes observed in the fruit-fly is given in T. H. Morgan’s “Evolution and Genetics.” (Princeton Univ. Press, 1925.)

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The evidence of evolution has been read in the rocks and the structures of plants and animals, but under the miscroscope Dr. Jennings is able to follow evolution not as a theory but as a thing that is actually taking place.

Intensified study reveals that the hereditary characteristics do become changed by external conditions. Through such diversities, continuing for great numbers of generations, single stocks, uniform in their hereditary characteristics, gradually differentiate into many faintly differing hereditary features.

In higher organisms the state of knowledge of this point appears less satisfactory. But the evidence, so far as it goes, indicates that processes here are in agreement with those in lower organisms.

“The organisms whose bodies are condensed into a single cell have, too, a life condensed into a few hours. They present a wonderful opportunity for solving in a brief period some of the deeper problems of life.

“In a watch glass on our table we may in a week see generations come and go. We may follow in successive generations the struggle for existence and the results of natural selection.

“In a few days we may see the birth, babyhood, youth, and age of individuals and their replacement by descendants. We may study the inheritance of parental traits by the new generation or the appearance of new traits. We may observe how the population changes with the passage of ages—all while we wait for one of the changes of the moon.

“What have the simple organisms to teach us on youth and age, death and rejuvenescence, heredity, variation, evolution?”—Dr. Jennings.

“In scientific deductions one single divergency suffices to demolish the structure on which they are based.”

Evolution is not a force, but a process; not a cause, but a law.—Lord Morley.

1. For details as to this process of recombination of genes in reproduction from two parents, see any modern text-book of genetics; e.g., T. H. Morgan’s The Physical Basis of Heredity.