Book - Evolution and Genetics 12

From Embryology

Morgan (1925) Evolution and Genetics: 1 Different Kinds of Evolution | 2 Four Great Historical Speculations | 3 Evidence for Organic Evolution | 4 Materials of Evolution | 5 Mendel's Two Laws of Heredity | 6 Chromosomes and Mendel’s Two Laws | 7 Linkage Groups and the Chromosomes | 8 Sex-Linked Inheritance | 9 Crossing-over | 10 Natural Selection and Evolution | 11 Origin of Species by Natural Selection | 12 Non-Inheritance of Acquired Characters | 13 Human Inheritance | Figures

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Chapter 12 The Non-Inheritance of Acquired Characters

For more than a hundred years the question has been discussed as to whether habits and physical characteristics acquired by an individual during its life are transmitted to its children. Lamarck's theory of evolution rests on the assumption that adaptations in the animal kingdom are brought about in this way. Although Darwin once referred contemptuously to Lamarck's nonsense, which he understood to imply that adaptation results from the slow willing of animals, he later accepted a view that is in all essential respects really the same as Lamarck's. In fact, Darwin went even further than Lamarck in attempting to explain by means of his hypothesis of pangenesis how changes in the body might be transmitted to the reproductive cells and reappear in the offspring.

Despite the high authority of Darwin's name there has been a steady falling away from this belief among biologists trained in modern methods of experimental research. It is true that among stock breeders and farmers there has always been, and there is still, a widespread conviction that acquired characters are transmitted, and in the folklore, both ancient and modern, of many peoples there are myths that turn on a belief in the inheritance of such characters. Phaethon driving the chariot of the sun over Africa lost control of his father's horses and coming too near the earth, "it is said the people of Aethiopia became black because the blood was called by the heat too suddenly to surface," and they are black to this day.

1 From The Yale Review, July 19:24.

The palaeontologist Cope, an ardent Lamarckian, relates a storv "from that keen observer" Professor Eugene W. Hilgard, describing the origin of the twisted tails of the cats in his neighborhood. A female ("and very prolific") cat when half-grown met with an accident that produced a compound fracture. Her kittens inherited the maternal twist and found favor in the eyes of their master, described as "my Chinaman." Cope also relates the following anecdote on the authority of an educated and reliable breeder of game fowls : "A game-cock, in his second year, lost an eye in a fight. Soon after, and while the wound was very malignant (it never entirely healed) , he w^as turned into a flock of game hens of another strain. He was otherwise healthy and vigorous. A very large proportion of his progeny has the corresponding eye defective. . . . The hens afterwards produced normal chickens with another cock. Both strains had been purely bred for ten or more years and none of the fowls has been bhnd unless from fights."

The myths relating to prenatal impressions are the most pathetic of all the inventions of human credulity, and they are as old and as widespread as the inheritance myths to which they are closely related. Jacob's slippery trick with the rods will be long remembered. "And he set the rods which he had pilled before the flocks in the gutters in the watering troughs when the flocks came to drink that they should conceive when they came to drink. And the flocks conceived before the rods and brought forth cattle ring-streaked, speckled, and spotted." The world is today filled with old wives' tales of prenatal influences. These mysteries, the ill-begotten offspring of ignorance, have contributed their baneful share to the social inheritance.

It is a strange commentary that, while zoologists have never met with much success in their endeavors to trace the origin of structural changes to the inheritance of acquired characters, numerous proposals have come from physiologists and psychologists. There was some consternation in 1923 when the great Russian physiologist, Pawlow, reported the results of experiments that go far beyond what most Lamarckians have dared hope. Pawlow's conclusions — and as yet we have only his conclusions — are very surprising. They can best be given in his own words :

The latest experiments (which are not yet finished) show that the conditional reflexes, i.e., the highest nervous activity, are inherited. At present some experiments on white mice have been completed. Conditional reflexes to electric bells are formed, so that the animals are trained to run to their feeding place on the ringing of the bell. The following results have been obtained :

The first generation of white mice required three hundred lessons. Three hundred times was it necessary to combine the feeding of the mice with the ringing of the bell in order to accustom them to run to the feeding j^lace on hearing the bell ring. The second generation required, for the same result, only one hundred lessons. the third generation learned to do it after thirty lessons. The fourth generation required only ten lessons. The last generation which I saw before leaving Petrograd learned the lesson after five repetitions. The sixth generation will be tested after my return. I think it very probable that after some time a new generation of mice will run to the feeding place on hearing the bell with no previous lesson.

Until we have a full account of Pawlow's methods it msLj be safer to wait before interpreting his results ; but this is by no means a new topic, for already the effects of training and its possible inheritance had been examined by three American investigators who used the most approved methods that experience has taught are essential in obtaining data of this sort. ]Miss Vicari' has carried out for two years a careful set of experiments with mice, extending over four generations. The records of each individual and its pedigree were kept. The outcome shows that no such effects as those reported by Pawlow appeared. MacDowell also carried out at Cold Spring Harbor extensive experiments on the possible effects of alcohol in inheritance as tested by ability to learn a maze, and, as a control, kept records of related rats that had been trained by the same tests used for the alcoholics. His data, recently published, show no improvement in the offspring of trained individuals over those not trained. Halsey Bagg^ has published significant data on mice tested in a maze, data that cover three generations, and here too there is no evidence of improvement resulting from training.

It may be objected that the methods employed were not the same as those used by Pawlow, and, that we must wait for his evidence. This is not to be denied; but, on the other hand, the American data warn us not to generalize as to the inheritance of training. Our human experience, too, teaches caution; for how simple would our educational questions become if our children at the sound of the school bell learned their lessons in half the time their parents required! We might soon look forward to the day when the ringing of bells would endow our great grandchildren with all the experiences of the generations that had preceded them.

2 Science, Vol. LIX, 1934, p. 303.

3 Science, Vol. IJX, 1924, p. 30-2. ^Archives of Psychology, Vol. XXVI, 1930.

The attempt to identify heredity w4th memory has been made over and over again. The most brilliant and irresponsible undertaking of this kind was that of Samuel Butler in his books on Life and Habit and on Unconscious Memory. His contention was, however, neither the first suggestion of the sort, nor was it to be the last. A few years before him a German physiologist, Hering, had elaborated this idea. Today this question has more than an historical interest, since the memory-heredity theory has never been without an advocate. Books continue to be written about it. Orr in this country advocated something of the kind, but was rather vague in his applications. Semon in Germany invented a full terminology, for his Mneme. Rignano in Italy attempted to give it a more physical expression, as indeed had Haeckel much earlier. Ward in England has spoken as a philosopher in its favor, and Bernard Shaw as a dramatist.

The comparison between heredity and memory has taken protean forms ; none of its advocates being able to do more than throw out suggestions as to what sort of "identity" they were talking about. Fantasy rather than prosaic science is the characteristic feature of all these theories.

That these speculations have produced ahiiost no effect on present biological thought is not surprising, for a moment's consideration will show that, at best, the basis for the comparison between memory and heredity rests only on a vague analogy. In each case something appears and reappears. In the one case, a memory of the past in the brain as we say; in the other case, a repetition of a similar type of behavior in successive generations. It is tacitly implied that because memory is a familiar process to us we must know more about it than about heredity. The fact, however, is that memory is one of the many obscure fields of human j^sychology. It is today more obscure to us than is heredity itself. Are we not justified, therefore, in looking askance at attempts to account for a phenomenon taking place in one realm of observation by an appeal to another, less w^ell understood? It is not an exaggeration to say that some of those who have propounded memory theories of heredity have never been in close touch with the facts of heredity and development that are familiar to students of these subjects. Our present knowledge of the relations of parent to offspring is so different from anything ever imagined by the memory advocates, that their speculations a^^pear to the zoologist as crude as they are often grotesque.

During the last quarter of the last century, one of the most important branches of biology came to fruition. The microscopic study of cells and eggs and their relation to development and inheritance, made great advances and cleared up many obscure questions. These observations were carried out in complete independence of the speculations concerning heredity that had gone before ; and the outcome has furnished a starting point for further interpretations that have led in our own time to far reaching discoveries. It is not possible to give here even a summary of the evidence, because its understanding requires familiarity with microscopic observations covering a very wide and unfamiliar field. But, in general, I may state that the work has led to the conclusion that the properties of the reproductive cells which are responsible for the characters of the body, are inherent in these cells ; and that the transmission of these properties is independent of the body-cells, and calls for no interference from them. This is summed up in the phrase "the isolation of the germ-plasm." The principal idea that this familiar phrase is intended to convey is exactly the opposite of that implied in the inheritance of acquired characters. The individual starts as an ^^^ which is itself a cell. The ^^^ divides and produces a vast number of cells essentially like itself. ]Most of these cells become changed, as development proceeds, into the tissues and organs of the body, but a few of them remain as the reproductive cells of the individual in which they live. Here they multiply to become each in turn the beginning of a new individual with its contained eggs. In a word, the egg produces the body — not the body the egg.

All this is now conceded by everyone familiar with the evidence ; but two further points are open to discussion. The first of these involves the possibility that the germ-cells may be affected by the vicissitudes of the body-cells, so that when their turn comes to produce a new individual they reflect in some way the changes that have been impressed on the body cells. If this takes place, the inheritance of acquired characters would not be incompatible with the cell theory although extraneous to the theory. The second point relates to the possibility that the changes in the external world that affect the body may produce a corresponding change in the germ-cells. No amount of argument or a priori reasoning is likely to settle these problems ; but fortunately there is at the present time a large body of evidence, and some of it experimental evidence, that is significant, and, I think, convincing. Here, if anywhere, we may hope to find proof on which to base a reasonable judgment of the situation. To this evidence, then, I propose to appeal.

The evidence is of various sorts, and may be roughly grouped under several headings. First, that of the supposed inheritance of use and disuse. This takes us back to Lamarck, but while he rested his case on generalities that were often fantastic, such as the origin of the giraffe's long neck, there is now a good deal of evidence that is significant and unfavorable. Darwin explained the eyeless condition of many cave animals as a result of disuse. Recently Pa3aie has bred fifty generations of flies in total darkness and has found that their reaction to light had been in no way impaired. Darwin suggested that the wingless condition of some insects living on islands was due in part to disuse. Now, there have appeared in our laboratory cultures of flies raised in milk bottles, of three different kinds having no wings. These appeared as single individuals with the wings entirely absent, from parents whose wings had not decreased visibly in size in their long confinement. Each of the new types arose by a mutation; and the inheritance of the wingless condition shows that they owe their peculiarity to a change in a single hereditary element, and are, in this respect, comparable to the four hundred other mutant types that have also arisen, whose new characters have no conceivable relation to their confinement.

It is more difficult to obtain definite information as to whether or not the use of a part that increases its size or improves its functions is inherited. Imaginary cases of this sort are abundant, but since other explanations will cover them they do not serve our present purposes. There are no measurements, so far as I know, to prove or to disprove the claim that the children of blacksmiths have stronger arms than other children, or that the children of football players have bigger legs.

William Brewer supposed that the speed of trotting horses was due chiefly "to better training but also in part to special exercise of function." Later Caspar Redfield insisted that the wisest sons have been born to the more aged fathers, and that the records of racing horses show that the fastest colts have come from parents that have been trained for racing; but his statistics will not stand the scrutiny of an actuary. Pearl has shown the fallacies that lie concealed in his premises.

The loss of a part is supposed in popular traditions to lead sometimes to its absence in the offspring. The typical example is that of the cat whose tail was pinched off by a closing door. Her kittens were tailless. There are, I believe, authentic cases of this sort, but it is also true that unpinched cats often have tailless kittens. In fact there is a special breed of these cats which when crossed to other cats transmit their peculiarity, and since from the nature of things the paternity of cats in general is always 023en to suspicion no great weight is to be attached to an occasional accident and the occurrence of tailless kittens — except in so far as it illustrates a curious faculty of the human mind to draw premature inferences. In rebuttal to the cat anecdotes it should be pointed out that some races of dogs and sheep have had their tails removed for generations and that puppies and lambs are born still with tails. Both Cope and Weismann cut off the tails of mice for several generations without producing bobt ailed mice. We do not have to go to the lower animals to get evidence. The several kinds of mutilations and removals that man has practised on his own body for centuries have left no permanent record on the race.

From removals to distortions is a distinct step, since it has been said by some of the Lamarckians when pressed for evidence of the inheritance of loss of parts, that, after all, the part is gone, and could not be supposed to transmit its absence. This evasion does not cover the case when a distortion is in question. The stock case is the flat fish, which, according to Cunningham, owes its asymmetry to the habits acquired by its ancestors that came to lie on their sides at the bottom of the sea. One eve was thereby put out of commission, but, as a result of the muscles pulling it over so that it could peep around the corner of its own head and look up, the eye slowly shifted "in time" until today it too lies on the side of the head that is uppermost — otherwise, of course, it would have been expected to degenerate.

We do not have to go to Eocene times for evidence. Chinese women of high caste have had their feet bound and deformed for many generations, and now that the custom is being abandoned the children do not appear to have feet different from those of other Chinamen. Nearer home we do not observe the effects of the corsets of our grandmothers on the size of the waists of our children.

Several years ago a famous French physiologist, Brown- Sequard, described some interesting facts about epilepsy and malformations in guinea pigs that he interpreted as due to the inherited effects of surgical operations. At the time, these experiments aroused great interest, and were much discussed, by zoologists at least. The operations have been repeated on rather a large scale and offspring obtained, but with results so inconclusive that Sequard's work is largely forgotten, and not often quoted by those who themselves have new^ claims to bring to the attention of the public.

If we turn now to the experimental evidence of more recent date, we shall find several instances where induced changes have led to deformities and malformations which may "reappear" in the next generation, and hence may be said, in a sense, to be inherited. But the storv they tell leads to a very different interpretation from the popular one of the inheritance of acquired character ; and while it is not entirely clear sailing, yet the general trend of the work is instructive and furnishes, I think, more than a hint as to the way in which some of these results may have been produced.

I refer to the experiments of Stockard on the influence of alcohol, of Guyer on the influence of antilens serum ; of Griffith and Detlef sen on the effects of long continued rotation; of Bagg and Hanson and Little on some of the effects of radium and of X-rays. To give a fair treatment of the interesting results that have come out of this work would require a detailed account of the special conditions involved in each case. To make a generalized statement that would cover them all would undoubtedly mislead the reader. I shall attempt, therefore, a compromise between these extremes.

Many of the facts can be accounted for on the view that the reproductive cells have been directly injured by the treatment, and since there is evidence that the chromosome mechanism is the basis for the transmission of the hereditary elements, one may even go further and suggest that the chromosomes have been altered. Now, embryologists have been familiar for a good many years with the injurious effect of alcohol, of X-rays, and of radium on the chromosomes in causing irregularities in their distribution, and with the consequent injurious effects on the developing embryo, so that one need not go far afield to find evidence in support of the view that injuries produced on the germ-cell may affect the individual that comes from it. How far the injuries induced by these agents are specific, and how far general is difficult to state at present ; but since, as Stockard has pointed out, the organs affected are just those that are most subject to injury when eggs are treated by many kinds of reagents it appears that the results are general rather than specific. The organs affected are the most delicate parts or the parts that require in their development the most perfect adjustments. I am also inclined to favor such a view, which, if established, may explain why alcohol, and X-ravs, and radium show their effects most often in the malformations of the eve.

The more difficult task remains to attempt to appraise those results in which a highly specific effect is claimed to have been produced. Guyer's experiment easily comes first in this respect. He removed the lenses from the eyes of rabbits, crushed them, and injected the mash into fowls. After a time the blood of these birds was injected into pregnant rabbits. The lenses of the offspring were often opaque and other abnormalities also appeared in their eyes. The effects were transmitted to later generations both in the male and female line. Here we have apparently a straightforward case of specific inheritance, unless, indeed, the injected serum is supposed to have affected not only the eves of the embryo but their germ-cells also. Crucial experiments w^ould settle this point, but as yet they have not been forthcoming. Guyer's experiment has been recently repeated by Finlev and also by Huxley and Carr- Saunders with entirely nee^ative results. We can safely w^ait, therefore, until further and more critical evidence is obtained as to the nature of the effect, if any, that was induced in Guyer's experiment.

The next best case is that of Griffith and Detlefsen. Rats were rotated for several months in cages. Some of the young born outside the cage showed irregularities in their gait, and when tested gave a different and specific response according to whether their parents had been rotated to the right or to the left. Detlefsen states that the disequilibrated rats showed frequent pathological sequelae, such as discharges from the ears; and this, he says, raises the question "whether Griffith has not merely presented us with numerous specimens of some vertebral disease." The disease once begun might be contagious, but he adds, "It is difficult to compromise this hypothesis with Griffith's contention of specificity."

This brings us finally to a point where something more definite may be said and therefore said briefly. Blakeslee and Belling have shown that if, during the maturing of the reproductive cells of a flowering plant, the common jimson weed, the plant is subjected to cold, the germ-cells may be so affected that the distribution of the chromosomes is on rare occasions altered, and a plant may be produced that has double the normal number of chromosomes. This change carries in its wake some corresponding changes of character. Changes of both these kinds often take place when the egg is not treated, and they are transmitted in the same wav, so that, at best, the special environment inducing them can only be said to make their occurrence more frequent.

Finally there is a considerable body of evidence showing that characters, whose development is known to be affected by environmental influences (which therefore might be supposed to be the very best kind of material to exhibit the effect of acquired characters) are not affected by the changes induced in their parents by the environment. There are several striking cases of this kind that have been met with in the course of our experiments with vinegar flies. There is a race of these flies that have been long inbred, in order to make them uniform in a genetic sense, in which the eves are entirely absent in most individuals, but occasionally one or both eyes may be present much reduced in size. If the flies that have these small eves are bred to each other they arrive exactly the same results as when their eyeless brothers and sisters are bred together. As each stock culture gets older, more and more of the flies that emerge have eyes, and, towards the end, an increased number of the flies have both eyes present and almost full size. If some of these are used as the parents of a new generation, the results obtained are precisely the same as when eyeless flies are used. What better evidence could we hope to obtain to show that the presence of a character in the individual has no influence on the reproductive cells in this case does not stand alone but is duplicated by similar evidence from other characters subject to environmental changes in these flies, namely, bar eyes, abnormal abdomen, and extra legs, all of which are greatly affected by the environment, but the effects are not transmitted. Is it surprising, then, in the light of these detailed and controlled data that we should look askance at claims which pretend to demonstrate the inheritance of acquired characters from observations that are in most cases inadequate to prove the point at issue ?

The experiments that Kammerer has carried on for several years relate, for the most part, to the kind of characters which I have just mentioned. He finds that salamanders spotted with black and yellow change to more black or more yellow individuals if kept on a black or a yellow background. Their offsj^ring reared on a neutral background show, he believes, some influence of the effects produced on their parents, and so on. Until these results are repeated on material that is more thoroughly controlled, or on material where the effect produced can be stated in measurable terms and not by pictures of selected material, it is in my opinion better to suspend judgment in respect to their interpretation. The careful work of Herbst that was undertaken to check up Kammerer's evidence has so far found no justification for Kammerer's conclusion. Much of the other work that Kammerer has brought forward as evidence of the inherited effect of the enviroiiiiieiit is open to the same objection — the inheritance of color changes in lizards, the change in the breeding habits of the midwife toad, and the development of horny pads on the thumbs of the male. That the environment causes changes in some of these characters need not be questioned, but that the effects produced are transmitted to the next generation, through the bodily changes produced, may be questioned, both because of the inadequacy of the evidence and also because in other cases where the materials are suitable for making such tests there is no evidence that such influences produce such results. Perhaps the most careful and thoughtful piece of analytical work that has been done in this field is that by Sumner, extending over five years, on the effect of heat and cold on the length of the tail, ears, and feet of white mice, as well as on the increase in the thickness of the hair in the cold.

Some of the mice were reared from birth in a cold room, others in a warm room. The average difference in temperature was eighteen degrees centigrade. The tails of the mice in the warm room series were longer than the tails of those in the cold, for mice of the same body length. The length of the feet and of the ears was also greater in the warmer room, although the effect of the cold on the ears was inconstant. These two kinds of mice were then brouglit together in a common room of intermediate temperature, where each series was bred separately, and measurements were made of the offspring when the mice were full grown. It was found that the tail, foot, and ear length were greater in the mice of warm-room parentage than that of cold-room parentage but the difference was not so great as that between their parents.

This may be interpreted to mean that the smaller increase shown by the tails of these mice of the second generation from warm-room parents was due to the intermediate temperature in which they were reared, while their length, which was greater than that of the mice of cold-room parentage, was inherited from the warm-room parents. But how? Was it the effect of cold on the germ-cells, or did it come from the longer tails of their parents? It is not easy to imagine that the effect was due to the direct influence of the cold on the germ-cells since mice are warm blooded and maintain a nearly constant body temperature when adult, and as young mice they were kept warm in the nest and by the brooding of their mothers. JMust we then conclude that the germcells are so sensitiye to slight differences in the size of the organs of the body that the effects are shown in the next generation? If so, might we not expect that all individual differences would reappear in the characters of the offspring?

But this question, at least, has now been settled by Johannsen's brilliant analysis on the non-inheritance of individual differences that are called forth hv the environment. His experiments were carried out on material that was adequate to give a crucial answer to the question involved. In support of Johannsen's conclusion there is an extensive body of genetic evidence which can he interpreted as meaning that while much individual variability is due to minor genetic factors, and this is inherited, some individual variability is due to the environment and this is not inherited.

Is it possible, then, that Sumner's results were due to chance, in the sense that the two series happened to give the averages shown ? It does not seem probable that this w^as so, but we can never be certain until the experiment is repeated on material that is first made pure for factors involving the length of the parts to be studied. Sumner is himself very cautious in his interpretation of his results. He says, "At no time have I declared my results to be proof of, or even evidence for, the inheritance of acquired characters. Indeed, I have insisted that in the present state of our problems this latter expression has become hopelessly obsolete. As regards the various possible interpretations of my own results I have always expressed indecision."

Castle and Phillips performed an experiment on guinea pigs that would be expected to show the influence of the body on the germ-cells if such effects are possible. The ovary from a black female was transplanted into a white female whose ovary had been removed. After the transplanted ovary had established itself, the white female was bred to a pure white male. The offspring were black, although the mother and the father were white. The eggs had not been affected by their sojourn in the body of a white individual.

It is not as widely known as it should be that most of the assumptions of the Lamarckians contradict the fundamental principles of Mendel's law of heredity. Mendel's law of segregation states that the hereditary elements received from the parents separate in the germ-cells of the offspring without having affected each other, and, by implication, without having been affected by the character of the individual in which they were contained. An example will make this statement clearer. Suj^pose a white mouse is bred to a wild gray mouse. The hybrid offspring will be gray. If two such hybrids are bred together they give rise to gray and to white offspring in the ratio of three grays to one white. This ratio is understandable if in the hybrids half of the reproductive cells carry the element for gray and half that for white. Thousands of instances of this sort are known today. To reject the evidence would be scientific suicide; to refuse to accept the theory would be to throw reason to the winds. Mendel's postulates concerning the clear separation of the elements of heredity mean that the white-producing elements in the gray hybrids have been unaffected by the gray color of the hair of the animal that carries them.

JNIan}^ similar types of inheritance in man tell the same story. A blue-eved man marries a brown eyed woman, and if she has come from a race pure for brown eyes, all the children will have brown eyes. If an individual of this parentage marries another with a similar parentage their children will be brown-eyed and blue-eyed as three to one. Still another case may seem more impressive since the character involved is one that dominates the normal and may appear therefore as something more positive in its nature. There is a tyj^e of malformed hand in which the middle segment of each finger is missing. If a short-fingered man marries a normal handed woman half of the children will have short-fingered hands and half of them will be normal. The explanation here is the same as before. The man was a hybrid (his father had short fingers and his mother was normal) , hence he produces two kinds of reproductive cells. When he marries a woman w^hose reproductive cells are normal, two kinds of offspring are expected and two kinds are found. It may be added that the normal children show no trace whatsoever of the influence of the hand of their shortfingered father and never transmit this deformity to their descendants — the separation of the elements in the dominant parent has been clean.

It is scarcely necessary to elaborate this theme. The facts are not disputed by any student of genetics who is familiar with the evidence ; and they furnish, in my judgment, convincing disproof of the loose and vague arguments of the Lamarckians.

The "will to believe" in the inheritance of acquired characters is widespread and an interesting feature of human behavior. The eagerness with which each new claim is listened to is only too familiar to those who concern themselves with evolutionary controversies.

The willingness to listen to every new tale that furnishes evidence of the inheritance of acquired characters arises perhaps from a human longing to pass on to our offspring the fruits of our bodily gains and mental accumulations. While every scientific investigator has sympathy for this human weakness, he cannot allow it to influence him in his examination of the facts as they actually exist. In our hope for the best we forget that we are invoking a principle that also calls for the inheritance of the worst. If we cannot inherit the effects of the training of our parents, we escape at least the inheritance of their misfortunes. A receptive mind may be a better asset for the child than a mind weighted down from birth with the successes and failures of its ancestors.

Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Morgan (1925) Evolution and Genetics: 1 Different Kinds of Evolution | 2 Four Great Historical Speculations | 3 Evidence for Organic Evolution | 4 Materials of Evolution | 5 Mendel's Two Laws of Heredity | 6 Chromosomes and Mendel’s Two Laws | 7 Linkage Groups and the Chromosomes | 8 Sex-Linked Inheritance | 9 Crossing-over | 10 Natural Selection and Evolution | 11 Origin of Species by Natural Selection | 12 Non-Inheritance of Acquired Characters | 13 Human Inheritance | Figures