Popular Science Monthly/Volume 82/January 1913/The Inheritance of Acquired Characters
|THE INHERITANCE OF ACQUIRED CHARACTERS|
THE variability of animal bodies is a very evident fact. The individuals of every species show variety in color, form and size. Three types of variability have been discovered; fluctuating variation obeying the laws of chance, mutation appearing as sudden loss or gain of a color or other feature, and acquired characters gained by an individual in relation to its surroundings. Among these three types are sought the great factors of evolution. It is a singular fact that no great biologist has attempted to use all three of these factors as the basis of his system, but each author has sought to build his hypothesis upon some one all-important factor.
Fluctuating variation is undoubtedly the greatest of these factors in the part it has played in the history of evolution. It was made by Darwin the corner-stone of his theory when he claimed that natural and artificial selection could produce almost unlimited effects by the elimination of all but the most favorable among thousands of variants in a species. In the debates over the general theory of evolution there has been no argument more often used than the plausibility of Darwin's theory of the survival of the fittest. The public, in accepting the truth of the theory of descent, has come to look upon this factor of fluctuating variation as a necessary part of evolution. In fact, to many professional biologists Darwinism has become synonymous with the survival of the fittest variations.
The theory of mutation is the most serious opponent of the Darwinian theory of selection of variations. Based at first on the evidence gathered by De Vries, it has grown in popularity with the growth of the knowledge of the inheritance of unit characters, and with the discovery of pure line inheritance. In the minds of many biologists it has the advantage of showing a method of rapid evolution more or less independent of the guidance of natural selection. The more ardent supporters of the theory have claimed for it the position formerly held by the theory of fluctuating variations, trying to show that all evolution must be in the nature of loss or gain of unit characters.
That the familiar acquired characters of animals should be inherited was once taken for granted, and, in fact, is still a general belief in the world at large. This theory was held by Lamarck to be a great law of evolution. It was defended by Spencer, and assumed occasionally even by Darwin. In the light of careful experiment, however, it has been largely discredited. The verdict of "not proven" has been pronounced against it, and many biologists would go even further and claim with T. H. Morgan that the theory was "unnecessary." Yet, not content with such a verdict, a small number of workers have persisted in their attempts to establish the theory of acquired characters as one of the essential factors of evolution.
Recently the discussions of evolution have begun to take a new turn. The old attempt to find one single all-important factor is being abandoned for a broader point of view that allows the possibility of many factors, some of them perhaps still unknown. V. L. Kellogg has pointed out the smallness of the number of observed mutations on which to base a comprehensive theory. Castle, a strong believer in mutation and unit characters, has affirmed his belief in the efficacy of selection in the production of new forms. Nowhere in the literature of the last year or two can be found any very dogmatic claim for a single allimportant factor which will serve as the basis for all kinds of evolution.
In this new atmosphere Lamarck's theory again receives serious attention, but not in its old form. To-day no one ventures to cite such examples as Spencer's famous illustration of the puppy that inherited from its mother the trick of begging for food. Such experiments as breeding away the wings of flies in small tubes, or breeding away the eyes of flies in dark chambers, attract but little attention. No great biologist is giving much time to experiments testing the inheritance of mutilations. On the other hand, there are many experiments to test the inherited effect of starvation, to test the effect of the application of chemicals directly to the germ plasm, and to test the effect of the application of extremes of temperature to animals with ripe germ cells. Several investigators have shrewdly seen the value of working with plastic types of animals like the amphibians, which present striking examples of dimorphism such as are found in axolotyl, Diemyctylus and various frog tadpoles. In this field a prominent worker is Kammerer, a representative of a school of experimental evolution in Vienna.
A short summary will be given of his researches on toads, tree frogs and salamanders. A few selected experiments will show very well the nature of the most recent work on the inheritance of acquired characters.
Kammerer in his work on the toad, Alytes, tried to prolong the tadpole stage until sexual maturity. He exposed the young tadpoles to a number of conditions such as darkness, cold, perfectly still water, each of which acting by itself tended to prolong the larval period. By exposing tadpoles to all of these conditions acting at the same time, he succeeded in producing one sexually mature female with the usual form of a tadpole but with mouth, legs and sexual organs of an adult toad. This one example was mated to a normal male. The progeny at the time the report was made, while not yet sexually mature, had been living in normal surroundings for six months longer than the usual larval period. Evidently the prolongation of the larval period had an inherited effect, and the new character was apparently a dominant factor. Strangely enough no inherited effect was seen in the offspring of those tadpoles which left the water before sexual maturity. Evidently the stimulus, whatever it may be, must act on the mature germ cells to produce an effect.
Other experiments were tried on this same toad with the object of changing its peculiar instinct of caring for its young. The male, under normal conditions, plants the fertilized eggs on his back and carries them there until the embryos have reached a stage just prior to the appearance of the fore-limb buds. The tadpoles are then liberated in the water. This peculiar instinct was found to be easily modified by change of surroundings.
The combined action of heat, dryness and darkness produced an egg called by Kammerer "a giant egg." The embryo from such an egg at the time of liberation was much larger than the normal type, fully twice as large, with well-developed hind limbs. Upon leaving the water the larva produced a small adult, a change in size due apparently to lack of water in the tissues. The new form of adult laid fewer eggs, which were larger and richer in yolk. Such eggs under normal conditions produced tadpoles which, in size and form at the time of hatching, were about half way between the old type and the derived type. The new character, then, was partly inherited. The stimulus in this case clearly did not act directly on the mature germ cells, but, if the dwarf form of the adult was due to lack of water in the tissues, there may have been an indirect action on the germ cells. The effect of keeping the eggs enclosed in their envelopes on moist earth for a considerable period of time produced a type of larva called by Kammerer "a land larva." This new type when placed in the water in its usual environment appeared superficially like a water larva of the same age, but a closer examination showed that the new conditions of development on land had accelerated the growth of the lungs. The land larva had lungs with well-developed air cells, while the water larva had simple sac-like lungs. The inheritance of the newly acquired character was evident, in that the embryos of the second generation could be kept on land for a much longer period before they began to show any ill effects from their unusual environment. Thus there was, according to the author, a progressive adaptation to land life through the inherited effects of environment.
In the presence of a relatively high temperature, the mature toads were constantly in and about the water, and in the breeding season mated in the water. The egg envelopes at once swelled up, and it was impossible for the male to plant these "water eggs," as the author calls them, on its back. Therefore the early stages developed in the water as is the case with other amphibians. The habit of mating in the water became fixed, and persisted after the removal of the artificial conditions of temperature. The eggs, meantime, at each successive laying became smaller and smaller through the loss of yolk. The larvæ hatched at a much younger stage than the larvæ from normal eggs. The adults reared from the water eggs mated in the water at the first breeding season, even under normal conditions of temperature. Succeeding generations showed intensification of the new characters in the decrease of yolk, and also in the development of more gills, which changed in number from one pair to three pairs. There was, therefore, as in the preceding case, an apparently progressive adaptation to environment through the inheritance of acquired characters.
The effect of this change on the germ plasm was tested by a cross between the old type and the derived type. The new character, as judged by the instinct for mating in the water, behaved like a dominant Mendelian factor. Dominance, however, was of an unusual kind. The male, whether of the old or the new form, impressed its character on all the offspring of the first generation, but the second filial generation showed the usual kind of segregation of characters in the ratio of three individuals of the dominant form to one of the recessive. Clearly the unexpected feature in the behavior of the factors in this crossing lies in the peculiarity of the sex-limited potency, not in the isegregation of factors. The most interesting fact in the experiment is the attempt to prove a change in the germ plasm by the modern method of applying the test of cross breeding.
Another series of experiments was tried on the tree frog, Hyla. This frog lays its eggs in the water in bunches of 800-1,000, enveloped by the usual coats of gelatine. A number of frogs were kept away from the water, but were allowed to crawl about on a water plant which held small amounts of moisture in the bases of its leaves. During the mating season the frogs deposited their eggs in the moisture on the leaves, according to a habit which is common among some of the tropical representatives of the genus. The young remained in their envelopes until the gills had become enclosed, whereas the young under normal conditions begin a free swimming life before the gills appear. A new type of adult was produced marked by its small size. These dwarfs when reaching maturity laid their eggs in water after the usual manner. The new habit was not inherited. The offspring of the dwarf frogs, however, had external gills at the time of hatching, a stage half way between the old and the derived type, and, moreover, they grew into adults of a size half way between the two types. This experiment, therefore, showed results very similar to those shown by the experiment on Alytes.
A third series of experiments was tried on two European salamanders, Salamandra ater and Salamandra maculosa. The former is a black mountain salamander which has the peculiar habit of bringing forth its young alive, always a brood of two with lungs already functional. The embryos pass through their early development in the body of the mother, nourished by the yolk of eggs that fail to develop. Salamandra maculosa is a yellow-spotted salamander of the lowlands which lays its eggs in running brooks. When kept away from the water the female of the spotted salamander at first dropped her eggs on the ground directly after fertilization. Such eggs failed to develop. In the course of two years, however, this salamander gradually acquired the habit of holding the eggs in the body for several weeks. The eggs became fewer in number and larger in size until the young were brought forth alive in a condition like that of the black salamander. Females of the latter were treated in an exactly opposite way. They were kept in or near the water until they acquired the habits of the spotted salamander. Inheritance was imperfect in each case. The new type of spotted salamander, under the usual normal conditions, deposited in the water a brood of five fairly well developed young. The new type of black salamander, under normal conditions, deposited in the water a brood of three young in a stage of development more advanced than that of the spotted salamander. When the artificial conditions of the experiments were continued through two generations the effect was greater. The author claims that his experiments show the inheritance of acquired characters influencing structure and instinct.
A second experiment was tried on Salamandra maculosa to test the inheritance of acquired color due to change of background. A brood of young salamanders was divided into two lots, one of which was kept for six years on a background of yellow, the other on a background of black. The former showed a decided increase in yellow markings, the latter an increase in black markings. The young of the yellow type were allowed to begin their development on a background of neutral tint, but before reaching maturity the brood was divided as before into two parts and placed, one part on a yellow background, the other on a black background. The set on yellow, after two years, showed a great increase of the yellow markings as compared with their parents, in fact the yellow pigment nearly covered the body. The set on the black background showed more black than their parents, but less black than the previous set similarly treated but of normal parents. These experiments, according to the author, show the progressive effect of environment in the inheritance of acquired colors.
The evidence presented by these experiments, which have been briefly described in the preceding paragraphs, should be considered in the light of the most recently discovered principles of heredity. A very important conception in this connection is the continuity of the germ plasm, another is the variability of the potency of unit characters.
Admitting, then, that certain acquired characters have actually appeared in later generations, we should consider, first, whether or not the germ plasm has been changed by the stimulus which has produced the changes in the body. It has been shown that starvation in the larval stages of insects will produce dwarfs in later generations, but here it is assumed that the unfavorable conditions surrounding the germ plasm persist and that there is no real change in the composition of the germ plasm. Can Kammerer's results be explained in the same way? Of course a Lamarckian can not be asked to produce a form which will not revert. The only test that can be readily applied is that of Mendelian inheritance. It has been shown by the author that in one case at least the new factor behaved like a Mendelian factor. Tower also found this true in crossing a pale potato beetle, which he derived experimentally, with a beetle of the normal color. Such a test to discover a change in the composition of the germ plasm is certainly very significant.
Granted, then, that the germ plasm has been changed, we should next consider whether it has been changed directly or indirectly. The experiment of keeping tadpoles in water for an abnormally long time showed that in order to affect the next generation the stimulus must continue to act until the sex cells are mature. Tower also came to the same conclusion in his experiments on the potato beetle where heat was the stimulus. The changes, then, are probably due to the direct action of chemical and physical stimuli on the germ plasm contained in the ripe germ cells, exactly as MacDougal produces mutations, as he claims, by injecting chemicals into the ovary of a plant. But why should the stimuli not effect the germ plasm of the embryo as well, since, according to the theory of continuity, the same plasm is always present even in the youngest stages? It may possibly be claimed that, if any such effect is produced in the embryo, the change is repaired before reproduction takes place.
Granted, then, that the germ plasm in these cases is more or less directly affected by the environment, we should consider whether the change is more than a change of potency of a factor already present. According to Castle such potency may be increased by selection. Perhaps the new environment may increase in some way the potency of a factor which is present in a weak condition. For example, in the case of the spotted salamander, the potency of the factor represented by the yellow pigment may possibly be changed by the action of the yellow light, which actually increases the amount of the pigment in the body of the adult until perhaps the nature of the fluids of the body cavity are affected and hence the germ cells themselves. Certainly such interpretations, while the merest speculations, are hard to deny from the known facts.
In such theoretical discussions of the nature of germ plasm and the potency of factors biologists are very apt to lose sight of the true historical purpose of the hypothesis of the inheritance of acquired characters. The real question to be answered first is whether or not acquired characters actually appear in following generations to such an extent as to make real contributions to the course of evolution. Even if the so-called inheritance is really a change in potency due to the direct action of stimuli on the germ plasm, nevertheless, the Lamarckian factor may be a real factor. We have not explained away any process by showing the method of its operation. The real question to be decided should be stated broadly. Do new habits and new environment produce changes in form which are of importance in organic evolution? "While a final answer can not at present be given to the question, it may safely be stated that a renewal of interest in Lamarck's factor is justified by the results of recent research.