Individual == egg + non-parental environment; but
for most mammals, including man--
Individual == egg + intra-maternal environment + non-parental environment.
This condition in mammals introduces a complicating factor which is likely to obscure the whole issue unless we bear it constantly in mind. In other words, we must discriminate sharply, in the discussion of inheritance in man, for instance, between two cla.s.ses of influences which may exist in the infant at birth, that is, which are _congenital_; namely, those which were truly inherent--were in the germ-cells--at the very inception of the young individual, and (2) those which might later have been derived from either parent by the yet unborn offspring. The latter are not regarded as truly hereditary. Since certain diseases or their effects belong here we occasionally find a physician using the term inheritance for such prenatal influences, but the more careful ones now employ the term _transmission_ to discriminate between such conditions and true inheritance. In its biological usage inheritance always refers to germinal const.i.tution and never to any condition that may be thrust on a developing organism before birth. It is clear, then, that congenital conditions are not all necessarily cases of inheritance.
=Three Fundamental Questions.--=To get at the question of the inheritance of body modifications with the least confusion, let us examine it in the form of three fundamental questions, as follows:
1. Can external influences directly affect the germ-cells?
2. Can external influences, operating through the intermediation of the parental body, affect the germ-cells? If so, is the effect a specific and a permanent one which persists in succeeding generations independently of external influences similar to those which originally produced it? Only such a condition as this would rank as the inheritance of a somatic modification.
3. Can the appearance of new characters be explained on any other ground, or on any more inclusive basis, than through the transmission of somatic acquirements, or do organisms possess heritable characters which are inexplicable as inheritance of such modifications?
Obviously the only way the question can be settled is through careful experimentation in which all possible sources of error have been foreseen and guarded against. Much experimental work has been undertaken for the solution of this problem as the goal and we may therefore select typical ones of these experiments and apply the results toward answering our three questions.
=External Influences May Directly Affect the Germ-Cells.--=There is evidence that under special conditions external influences may in certain organisms affect the germ-cells, but that this occurs commonly is extremely doubtful. For example, Professor MacDougal, by treating the germ-cells of the evening primrose with various solutions, such as sugar, zinc sulphate and calcium nitrate, has apparently succeeded in producing definite germinal mutations. He injected the solution into the ovary of the flower the forenoon of the day at the close of which pollination would occur. He reports that in this way changes were produced in the germ which found expression in new and permanent characters.
Professor Tower has experimented for a number of years with various species of _Leptinotarsa_, the potato beetle. By varying the conditions of temperature, humidity and atmospheric pressure when females were laying their eggs, he reports having produced variations in the young which came from these eggs although the mothers themselves were not changed.
According to Professor Tower slight increase or decrease in these environmental factors stimulated the activity of the color producing ferments, giving rise to melanic or darker individuals. Greater increase or decrease, inhibited them and produced albinos. He found also that at times the same stimulus might show different results in different eggs.
The effect, therefore, is a general and not a specific one. Ordinarily the eggs of these beetles are laid in batches. When one of these batches was laid and left under normal conditions, the usual form of young hatched from it, but other batches from the same female under abnormal conditions resulted in the production of atypical forms. For example, a normal two-brooded form became five-brooded. The commonest modification was the production of various color types. These once established, according to Professor Tower, behave as independent, inheritable units.
The experiments of Doctor Bardeen with X-rays and of others with X-rays, radium and other agents on the sperm and ova of amphibia show that these are very susceptible to injurious influence at or near the time of fertilization.
=Such Effects Improbable in Warm-Blooded Animals.--=However possible it may be to bring about germinal changes in invertebrata or lower vertebrata by such external agents as temperature and the like it is obvious that the probability of such extrinsic influences affecting the germ-cells of warm-blooded animals is very remote indeed. In the latter the germ-cells are more or less distant from the exterior and are at practically a constant temperature. Such experiments, therefore, beyond showing the possibility of producing changes in germ-cells, do not have very direct bearing on the problem of how inheritable variations are produced in man.
In his case about the only avenue of approach through which germ-cells might be influenced is the blood or lymph.
=Poisons in the Blood May Affect the Germ-Cells.--=Any poisonous material in the latter might injuriously affect the gametes. We know, in fact, that such poisons as alcohol, lead and various drugs, and also the toxins of various diseases, do so affect germ-cells. It seems plausible to suppose that changing conditions of nutrition may affect the const.i.tution of the germ-cells and thus induce changes in the organism which arise from these cells, but such nutritional effect is not yet a matter of established fact.
=Difficulty of Explaining How Somatic Modifications Could be Registered in Germ-Cells.--=As to our second query concerning the possibility of affecting the germ-cells through the intermediation of parental tissues, it is evident at a glance that since the germ-cells are built up along with the body and are not a product of it (Fig. 2, p. 13), if such effects are possible they must take place through the agency of some transporting medium. The germ-cells, being lineal descendants of the original fertile germ or zygote, already have the same possibilities of developing into an adult that the zygote had, and so the problem becomes one of modifying a complete germ already organized rather than of establishing a new germ by getting together samples of every part of the body. This is all the more evident when one realizes that usually the germ-cells are set apart long before the body becomes adult, that is, before the body has developed most of its characteristics. Moreover, among lower animals many instances are known where the immature young or even larvae will produce offspring which nevertheless ultimately manifest all the structures of the adult condition.
But supposing specific modifications of the germinal mechanism were possible, it is difficult to comprehend how an influence at a distant point of the body could reach the germ-cell, to say nothing of the even greater difficulty of understanding how it could become registered in the germ in a specific way as affecting a particular part. For it must be remembered that the organs of the adult do not exist as such in the germ but are present there only as potentialities. How, for example, can a change in the biceps muscle of one"s arm be registered in a germ-cell in which there is no biceps muscle, but merely the possibilities of developing one? Or how can increased mental ability which is contingent on the elaboration of certain brain-cells be impressed on a germ which has no brain-cells but only the capacity under certain conditions of producing such cells? For the brain of a child is not descended from the brain of his parent, but from a germ-cell carried by that parent.
=Persistence of Mendelian Factors Argues Against Such a Mode of Inheritance.--=On the face of things, the apparent inviolability of Mendelian factors which may remain unexpressed in the germ for one or many generations--indeed the whole matter of genotypical differences in the gametes of the same individual--shows the improbability of somatic interference with the germ-plasm. But notwithstanding this, because of the great importance of the issue, it is well to review in some considerable detail the various phases and possibilities of the question.
=Experiments on Insects.--=Some of the attempts to secure evidence of the transmission of personally acquired parental modifications in insects are very interesting. Many insects in the larval stages, particularly just after pupation seem to be especially susceptible to external influences.
They have been much used, therefore, for purposes of experiment. It has long been known that differences in size, in color and even in the shape of wings can be produced by various agents if applied at this period of development. From the standpoint of heredity, however, the important consideration is to determine if these experimentally induced changes have been reflected on to the germ-cells so that they reappear in the offspring of the modified individuals.
It has been found that in some cases where male and female are of different color, the color of the female can be changed to that of the male by altering the conditions of temperature. In certain cases types can be changed by cold so that they resemble varieties of the same species found farther north, and by heat, varieties found farther south. But not all individuals of a given lot are affected, and often different individuals of the same kind show different effects. Moreover, in some cases the same aberrations were produced by heat as by cold. This indicates that it is not so much a question of specific effects as a general physiological change, apparently mainly a matter of direct influence of temperature on the chemical composition of the pigments. The Countess von Linden in fact has shown that the extracted pigments can be made to undergo the same changes of color in a test-tube by heat and cold as in the pupae. But there is no evidence that the germ-cells of the living insect were affected in a specific way. In a small fraction of the offspring of such modified individuals abnormalities appeared, but these were not always of the same kind as those which had been produced in the parent. That is, there was no evidence of a trait or character having been acquired by the body and handed on to the germ-cell. Where an effect was produced on the germ-cell it was probably produced directly as in the first cases discussed.
Size, colors and markings of b.u.t.terflies have also been altered by subjecting the caterpillars or the pupae to such influences as strong light, electricity, various chemical substances, centrifuging, diminished oxygen supply, etc., but the results were in the main confined to the immediate generations. In the few cases where permanent inheritable changes were seemingly produced they were more reasonably interpreted as the effects of direct action on the germ-cells than as examples of inherited somatic modifications.
Starvation experiments which resulted in the dwarfing of adult individuals have been performed on various insects, and while the dwarf condition may persist through one or two generations due to a diminished food supply in the eggs of the dwarf, the stock in question when returned to normal food conditions soon resumes its original characteristic size.
=Experiments on Plants.--=Many experiments have been performed with plants, inasmuch as they are particularly p.r.o.ne to become modified by changes of food supply, or climate. For example, plants which grow luxuriantly in a warm moist climate or a rich soil may become stunted and markedly changed if transplanted to a cold climate or a poor soil.
Naturally, their progeny will exhibit the same behavior as long as they are kept under the new conditions. Experiments carried on through numerous generations, however, practically all show that the germinal const.i.tution of the plants remains unchanged, for when their seeds are planted under the original favorable conditions of soil or climate, the plants resume their former habits of growth. Naegeli, for instance, who made a study of many varieties of Alpine plants, and who carried on experiments with many of them for years in the Garden of Munich, concluded that no permanent effects had been produced by the Alpine climate and conditions in plants from other regions which had come under its influence. A few botanists have claimed to have found that the changes produced by the Alpine climate have persisted for a generation or two and have then worn off. More recent experiments on various of our field grains which have been stunted and cut down in productivity by growing for a number of generations under adverse conditions show that they have not been permanently modified by such treatment, for they resume normal productivity and size when grown again under favorable conditions.
On the other hand, Lederbaur found that a common weed, _Capsella_, when transplanted from an Alpine habitat to the lowlands did not return to the lowland type of the weed, but retained certain of its Alpine characteristics. It is not clear, however, that this particular species during its long sojourn of many generations in Alpine conditions may not have undergone a series of germinal variations and have developed into a new variety or species quite independently of changes wrought in the germ by reflected somatic effects. Indeed, in face of the preponderance of other cases to the contrary, this interpretation would seem to be the more plausible one.
=Experiments on Vertebrates.--=In the vertebrates we may also find examples of various somatic modifications experimentally produced, but evidence of their inheritance is as difficult to establish as in the invertebrates. Let us examine a few of the more significant of these which are alleged by some to bear evidence of such inheritance.
By decreasing the amount of water in an aquarium Marie von Chauvin was able to transform the aquatic, gill-breathing salamander _Axolotl_ into the gill-less land form _Ambystoma_, heretofore regarded by systematists as a different species. Either of these forms when s.e.xually mature produces its like. The salamanders in question have both lungs and gills, but after a time the ones which are to be land forms lose their gills and become exclusively lung-breathers. What seems to have been accomplished then is the accelerating or forcing of normal natural tendencies already inherent in the organism instead of introducing something new into the inheritance by way of the soma. _Axolotl_ is in all probability merely a larval form of _Ambystoma_ which with high temperature and an abundance of water reproduces without advancing to the final possible stage of its life cycle.
=Epilepsy in Guinea-Pigs.--=Perhaps the most frequently cited case and the one in which the defenders of the idea of somatic inheritance usually take final refuge is that of Doctor Brown-Sequard"s guinea-pigs, notwithstanding the fact that no one has had convincing success in repeating the experiments and that the original results are apparently open to more than one interpretation. This experimenter rendered guinea-pigs epileptic by certain injuries to the nervous system. Epilepsy appeared in some of the offspring of these operated animals. He regarded this as an example of the inheritance of an artificially induced epilepsy.
An indirect loss of toes occurred in some of the parents as a result of the operations on the nervous system. Some of their young also had missing toes. However, as has been pointed out by various critics, guinea-pigs are strongly predisposed toward epileptic-like seizures, and the epilepsy in the young may have been merely a coincidence. Voison and Peron believe they have shown that in epilepsy a toxin is produced that may affect the unborn fetus. That is, the result might have been due to a poison derived directly from the mother. The experiments in fact show that it was mainly in the offspring of affected mothers that the condition appeared. Others maintain that we do not know the exact nature of epilepsy, that in some cases it may be the result of infection by disease-germs, and that Brown-Sequard"s cases may, therefore, have been merely the communication of a disease from parent to child. As to the disappearance of toes it is a well-known fact that rodents in particular are likely to gnaw off the toes of their young very soon after birth, and little credence can be put in a lack of toes in such young as cases of inheritance except under conditions of much more careful observation than existed in Brown-Sequard"s experiments. A fuller account of these experiments will be found in Romanes" _Darwin and After Darwin_, Vol. II, Chap. 6.
=Effects of Mutilations Not Inherited.--=Many experiments have been performed by investigators to determine whether or not the results of mutilation are transferred to succeeding generations, but so far only with negative results. Many such experiments have been unwittingly carried on for many generations, in fact, by breeders and fanciers, in the docking of horses, dogs and sheep, the dehorning of cattle and the like, yet no satisfactory evidence of the transmission of such conditions in any degree has ever been forthcoming. The mutilations or distortions of the human body through various rites or social customs also fails to yield any convincing examples. Foot-binding, head-binding, or waist-binding must be repeated in each successive generation to produce the particular type of "beauty" that results from such deformities. And lucky it is for man that injuries do not persist in subsequent generations, otherwise the modern human being would be but a maimed relic of past misfortunes.
=Transplantation of Gonads.--=An interesting experimental test regarding the effect of the body on the germ was made recently by Castle and Phillips with guinea-pigs. It will be recalled from the discussion on Mendelism that when a black guinea-pig is mated with a white one the offspring are always black. These experimenters transplanted the ovaries from a young black guinea-pig to a young white female whose own ovaries had been previously removed. This white female was later mated to a white male. Although she produced three different litters of young, six individuals in all, the latter were all black. That is, not a trace of coat-color of the white father or of the white foster-mother was impressed on the transplanted germ-cells or the developing young. Later experiments of the same kind by Castle and Phillips, with other varieties of guinea-pigs, have yielded the same results. The body of the mother, indeed, seems to serve merely as a protective envelope and a source of nutrition.
=Effects of Body on Germ-Cells General, Not Specific.--=As far as the evidence regarding the modification of the germ-plasm by the body is concerned, we must conclude then that while under special circ.u.mstances the germ-cells may be affected, the effect is general rather than specific and the result as seen in the offspring has no discoverable correlation with any particular part or structure of the parental soma. The effect is presumably of much the same nature as where the germ is directly affected by external agents. Where a new character or a modification of one already existing is produced by a given condition of environment, in our experience so far to have the same repeated in the offspring, a similar evocative condition must prevail in the environment of the latter. Or in other words the new character is not a permanent one which persists in succeeding generations independently of external influences similar to those which originally produced it.
=Certain Characters Inexplicable as Inherited Somatic Acquirements.--=It would require remarkable credulity, in fact, to believe that some of the most striking features about certain plants or animals could have been developed by means of the inheritance of somatic modifications. For example, many animals such as the quail, the rabbit, or the leaf-b.u.t.terfly are protectively colored. That is, they harmonize in color-pattern with their surroundings so closely that they are overlooked by their enemies.
But how can this oversight on the part of an enemy so affect the bodies and through them the germ-cells of such individuals as to develop so high a degree of protective coloration? Or how, indeed, could any of numerous adaptive structures which one can think of, such as the color or scent of flowers to lure insects for cross-pollenation, the various grappling devices on many seeds to secure wide distribution by animals, or the like, have been directly produced by use or disuse or by any variation produced in them by the agents to which they are adapted?
=The Case of Neuter Insects.--=A very instructive example of the improbability that great skill, highly specialized structures, or certain instincts are first developed in the parental body as the result of use and then pa.s.sed on to the offspring, is seen in the case of neuter insects. In bees, for example, there are three cla.s.ses of individuals: the drones or males; the queens or functional females; and the workers, which are neuter, that is, take no part in reproduction. The latter are really s.e.xually undeveloped females. The queen can lay either fertilized or unfertilized eggs. The latter always give rise to males. The workers gather the food, attend the queen, wait on the young, construct the comb, and in short perform all the ordinary functions of the colony except the reproductive. They have many highly specialized structures on various parts of their bodies for carrying on their many activities, as well as the very highly specialized instincts necessary to the maintenance of the colony. But now, complex and highly developed as these workers are, since they do not give rise to offspring, no matter how much experience or structural modifications they may acquire during their lifetime, it can not be handed on to another generation. Nor can they have come to their present highly organized state through such a form of transmission since they are not the descendants of workers but of a queen. Any new modifications that appear in the workers of a colony must therefore have their origin in changes which have taken place in the germ-cells of the queen, and not in the soma of some other worker. It has been argued that the worker has not always been infertile; that at a more primitive stage of the evolution of the bee colony every female was both worker and mother, and that individual somatic acquirements might therefore have been transmitted, but this argument can not hold for many of the instincts or features of the modern bee because these have to do only with the conditions of life which exist in the colony in its present form. It is obviously absurd to maintain, for instance, that all the highly specialized instincts incident to queen production, queen attendance and the like were functionally produced through usage before there was any queen to produce or attend, while on the other hand, the very necessity of queen production and maintenance is the outcome of the infertility of the workers. Some workers have been known to lay eggs, but as these are few in number and are never fertilized, which means if they develop they can only produce males, they can play no considerable part in inheritance.
ORIGIN OF NEW CHARACTERS
=Origin of New Characters in Germinal Variation.--=This brings us to our last query as to whether the appearance of new characters can be explained on any other or any more inclusive ground than that which infers that changes undergone by the parent-body are in some way registered in the germ-cells so as to be repeated in a certain measure in the body of the offspring. The answer to the question of how inheritable variations do come to appear in offspring if not through changes produced in the body of the parent, is uncertain; nevertheless most biologists believe that they do not have such a somatic origin but arise directly as germinal variations. Some would attribute them to the fluctuating nature of living substance in general. The instability of protoplasm is one of its striking characteristics. It is constantly being broken down and built up, or, in other words, undergoing waste and repair. Like all other protoplasm, that of the germ-cells must also undergo these metabolic changes and it is possible though not proved that in this give and take of substances small changes occur in their const.i.tution which find expression in the offspring as variations. As already seen, substances in the blood other than food may also affect the const.i.tution of the germ-cells.
=s.e.xual Reproduction in Relation to New Characters.--=Some biologists attribute great importance to s.e.xual reproduction as a basis of variation and the origin of new characters. They argue that the mingling of determiners from two different lines must produce many new combinations and expressions of germinal potentialities. Plausible as the argument seems at first sight no one has succeeded as yet in securing proof that absolutely new characters can be originated in this way. What seems to occur under such circ.u.mstances is merely a reshuffling or sorting of old unit-characters. Although innumerable permutations and combinations of these may be made which find new expression outwardly, this is obviously not creating determiners of new unit-characters in the germ-plasm. While many biologists would not deny the possibility or even the probability that the determiners of unit-characters may sometimes combine or influence one another so as to form actual permanent new characters, the proof of such performance is wholly lacking. On the other hand, there are not a few biologists who argue that s.e.xual reproduction accomplishes just the reverse of increasing the extent of variation or creating new characters; according to them it tends to annul exceptional peculiarities of either parent by throwing the offspring back to the average racial type. It is thus looked on by these advocates as a stabilizer which reduces the amplitude of variations instead of increasing them. As a matter of fact the two ideas are not mutually exclusive; s.e.xual reproduction may accomplish both of these ends. A limited number of observations and experiments have been made to test out the correlation between s.e.xual reproduction and variation, but they have so far been too few or too inconclusive to enable us to come to a satisfactory conclusion.
While we are uncertain about the method of origin of new characters the fact remains that they do arise in abundance as abrupt mutations or otherwise and become a part of the permanent heritage of a stock. It is clear that s.e.xual reproduction may be one important means by which a given new character which has arisen in one or a few individuals may become incorporated in the species at large. Through Mendelian combinations and segregations it would by cross-breeding be spread and gradually introduced into more and more strains of the general population.
=Why So Many Features of an Organism Are Characterized by Utility.--=Germinal variations are seemingly at first more or less. .h.i.t or miss affairs as far as utility to the organism is concerned. Useless variations, so long as they are not actually harmful, may persist and apparently be indefinitely inherited. However, a special premium is put on variations which happen to be useful for they help the organism to succeed in its struggle for life and since success in the world of life means not only mere individual survival but also the production of progeny, through this very means insured transmission to subsequent generations. It is probable that the very many useful features of any organism, that is, its _adaptations_, have thus been established. It is possible also that many variations which at their inception are indifferent may wax in strength in successive generations until they reach a point where they must become either useful or harmful. In the former case they would mean increased insurance of survival for their possessors, in the latter, elimination.
With such an automatic process as this operative in nature it is not astonishing that the main features of any organism are characterized by their utility to it.
=Germinal Variation a Simpler and More Inclusive Explanation.--=The gist of the whole matter regarding the source of new characters in offspring seems to be that the explanation based on the idea of germinal variation is in last a.n.a.lysis the simpler and more inclusive, and there is no alleged case of inheritance of parental modification, which can not be equally well explained as the result of a germinal variation. There are numerous cases which can not be explained as transmissions of somatic acquirements even if this transmission could be established in certain cases. So, many biologists argue, why have two explanations when one is sufficient, especially when the other has never been conclusively established as true in any case and is obviously untrue in certain test cases? The att.i.tude of most investigators is that of the open mind. While feeling that the weight of probability is very decidedly against the theory of the inheritance of somatic modifications, they still stand ready and willing to accept any evidence in its favor which when weighed in the balance is not found wanting.
a.n.a.lYSIS OF CASES
While s.p.a.ce will not permit extended discussion, in order further to fix the nature of the problem in mind as well as to exemplify the conditions that must be satisfied to form convincing evidence of inherited somatic acquirements, it will be well perhaps to a.n.a.lyze a few typical cases as they are frequently cited.
=Are the Effects of Training Inherited?--=Breeders and trainers very commonly believe that the offspring of trained animals inherit in some measure the effects of the training. Thus the increased speed of the American trotting horse is often pointed to as strong evidence of such transmission. According to W. H. Brewer, the earliest authentic record of a mile in three minutes was made in 1818. The improvement, approximately by decades, from that time was as follows:
During 1st decade after 1818, improved to 2:34 2nd " " " " " 2:31-1/2 3rd " " " " " 2:29-1/2 4th " " " " " 2:24-1/2 5th " " " " " 2:17-1/2 6th " " " " " 2:13-1/2 7th " " " " " 2:08-1/2
By 1892, the date of Professor Brewer"s publications (See _Agricultural Science_, Vol. 4, 1892) the record had reached 2:08-1/2. Since then it has been lowered still further.
On the face of it this looks like a good case of inheritance of training, and Brewer himself believed it such. If so this would mean that colts of a highly trained trotter would be faster than they would have been if their parent had remained untrained. It is impossible to get positive proof in the case of any trained horse since there is no way of establishing just how speedy the progeny would have been had the parent remained untrained.
If it could be shown that colts sired by a trotter late in life were on the whole faster than those sired by the same father when younger and as yet not highly exercised in trotting, then the facts might give some evidence of value, but unfortunately no such records are available.
On the other hand, even ignoring the fact that improvement in track and sulky are probably the biggest items in the shortening of records in recent times, _selection_ instead of inheritance of the effects of training will equally well account for any innate progress in trotting.
And since, as pointed out by Professor Ritter, there are even more striking cases of similar improvements in other fields, such as college athletics, where the factor of use-inheritance is entirely precluded, it is wholly unnecessary to postulate it in the case of the trotter.