Book - Evolution and Genetics 13

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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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)

Chapter 13 Human Inheritance

A large number of malformations in man have been shown to be inherited. In the medical literature there are hundreds of family pedigrees in which one or another defect appears in successive generations, especially when the stock has been rather closely inbred, or where the defect is a dominant one. The few books in which these cases of human inheritance have been collected may give the impression that our knowledge of man's heredity is mainly concerned with the transmission of his defects. The eugenic programme or recommendations with which these treatises usually wind uj) may give the impression that our chief concern with human inheritance relates to the elimination of the defective materials (cacogenics) that have become incorporated in the species, rather than with the discovery of superlative human materials, their preservation and perpetuation (eugenics) . All this calls for comment.

General Statement

In extenuation of the depressing effect of such literature it may be said that malformations are utilized for genetic work not because of their intrinsic interest — although to the pathologist they are in themselves important — ^but because, as in other species, they furnish clear-cut characters that are sharply separated from normal structures, and hence can be traced through successive generations. It may properly be claimed that in studying the inheritance of each defect we are also studying the inheritance of a normal character that forms the other member of the contrasted pair. This statement, however, calls for an important reservation ; for, all that we mean by such a contrast is that the "normal" is not the abnormal. We do not in reality know anything more than this. This relation is, however, inherent in all Mendelian contrasted character-pairs, unless members of an allelomorphic series are somewhat more specific.


The presence of malformations of the body in human stocks is not supposed to be due to a greater tendency in the human species than in other species to produce, de novo, defective mutants, but rather to be due in man to the preservation of individuals having such characters and allowing them to marry, or at least not preventing them from mating. The higher ethical standards of man lead him to preserve human life, and in the absence of severe competition (through which the maladjusted would go under) the defective child reaches maturity.


If the new character is recessive its gene may become widely disseminated in the human germ material before two individuals each with the recessive gene mate. One-fourth of their offspring will then show the defect. by the time this has occurred the gene may have become so widely spread that the most that can be done — if something is to be done — is to discourage the defective individuals that have reappeared from further contamination — a council of perfection that may not be appreciated.


When the defect is dominant it will appear in half the offspring[1] if marriage with a normal person occurs. The other half of the children that are normal do not transmit this dominant defect, and have, so to speak, escaped entirely from the inheritance.


There are, however, a number of cases where the defect is not perfectly dominant. This means that its variability is so wide that a few individuals that carry the gene may fail to show it, or show it to such a small degree as to escape casual examination. There are, apparently, cases of this sort which have been utilized by the opponents of the modern theory of heredity as justification for the statement that such a character does not show "strictly ]Mendelian inheritance." Unless suitable tests are made it is not possible to claim that extreme cases of this sort are exceptions to Mendelian inheritance, for, similar cases are known in other animals, and have been shown not to be exceptions, but due to the wide variability of the hybrid character.


Aside from the major physical defects there are many smaller ones that do not interfere seriously with the welfare of the individual, or which can he corrected by modern appliances or operations. We have far less information concerning the inheritance of these blemishes, and at present can seldom be certain that they are inherited or how they are inherited. Only by comparison with the better-known cases can we surmise that inheritance may play a role in many of them. Since the disadvantages that follow are slight, or can be corrected, these defects have little practical importance, which need not, however, detract from their theoretical interest. Other ways than elimination by means of artificial selection have been found to standardize individuals that show these slight departures. Corrective surgery has proven a more efficacious remedy in man than the slow process of selective breeding.


In genetic work each mutant type (defective or otherwise ) is contrasted with the original type from which it came, sometimes called the normal type. In man and in some of the domesticated animals there is no standard or original type with which to make such a comparison ; and opinions may even differ as to what is to be regarded as a normal type, each race having probably a standard of its own. The situation is the same in several domestic races of animals and garden plants. In some, the new mutant genes have entirely replaced the original genes, i.e., the race has become pure for certain mutant genes. Whenever the original genes have been replaced by mutant genes it is not possible to recover the old ty2)e, but if two races have independently arisen from the same original wild type, and have accumulated different sets of mutant genes, it is still possible to obtain the original type by crossing in so far as each has retained some of the original genes. For example, there are two white breeds of fowls which when crossed produce offspring show^ing the colored plumage of the wild jungle fowl. Similarly there are two white races of sweet peas which if crossed give the color of the purple Sicilian w41d pea from which our cultivated forms are said to have come. Some of the races of mankind have been long separated. It might seem possible to recover the type from which they have departed by crossing them. Racial crosses have been made frequently, and the hybrids described, but there is no way of determining how far the outcome represents in certain respects the common ancestral type, and how far it is due to the interacting dominant factors of the combination. Possibly it might be supposed that the mulatto, w4th a yellow skin, that results from the white-negro cross represents the type of skin color from which both white and the negro races have diverged. If so, a yellow race breeding true to that color might be obtained after the white and the black genes had been replaced by the original yellow genes. As yet there is no certain record of such a consummation, although the deficiency of white-skinned and black-skinned offspring from mulattos of later generations might in some few cases be accounted for in this way. On the other hand the intermediate color of the mulatto of the first generation might be due to the interaction of the incompletely dominant genes of the two parents, and in later generations it might not be possible to distinguish by inspection alone such a condition from that due to the restoration of the original normal genes. It would require elaborate genetic tests to settle such a question.


Meanwhile we shall have to rest content with the admission that there is no single type of human normal individual with which to standardize the different racial types. At best, cases of human atavism produced by crossing, would be expected to go no further back than the race to which the modern types of men converge, and from an evolutionary point of view this is a very recent event. We should anticipate, therefore, that all the races of mankind have an enormous number of genes in common and only few that are different. The latter produce the relatively slight structural differences that are found in different races.

The Inheritance of Physical Defects

A few examples of the inheritance of physical characters in man will suffice to show that in his inheritance man conforms to the same laws that regulate the inheritance of other animals and plants.

Morgan 1925 fig65.jpg

Fig. 65. Two Brachydactyl hands. (After Farabee.)


Morgan 1925 fig66.jpg

Fig. 66. An X-ray photograph of the bones of a Brachydactyl hand.


There is a rare abnormality of the hand and foot known as brachydactydy, or short-fingeredness (figs. 65, 66). Farabee has recorded the history of an American family of this sort, and Drinkwater that of a British family, and later of a second family that migrated from America back to England. As seen in figure Qo, the fingers of the brachydactyl hand are short owing to the absence of the middle segment of each finger. The fingers are about half normal length. A short-fingered man marrying a normal woman transmits the defect to half of the children. The character is dominant (jig. 67). There are no recorded cases of the marriage of two short-fingered persons and the pure (or double) dominant character is unknown. There is a possibility that such an individual might not be viable.

Morgan 1925 fig67.jpg

Fig. 67. Pedigree chart of the inheritance of Brachydactyly. (After Farabee.)


Drinkwater has also recorded other cases of minor brachydactvlv in which the finoers are less shortened. Several other cases in which one or more of the fingers are short have been descri})ed. One such case — also a dominant — has been traced by Mohr and Wriedt through six generations which carries the pedigree to the year 1764. Here the shortening involves mainly the forefinger (fig. 68) .

Morgan 1925 fig68.jpg

Fig. 68. Four pairs of hands showing a shortened condition of the fore-finger. (After Mohr and Wriedt.)


Color-blindness is a sex-linked recessive character in man. A color-blind man married to a normal woman has only normal daughters and sons; all of the daughters, however, transmit color-blindness to half of their sons (fig. 69) .


Color-blind women are rare, because they can never arise unless a color-blind man marries a w^oman who is color-blind, or else marries a normal woman who had a color-bhnd father, or had a mother heterozygous for color-blmdness (fig. 70) .


The pedigrees of color-blind families — and they are many — leave little doubt as to the mode of inheritance of this character (fig. 71).


Accepting this evidence as on the whole satisfactory, there is still something more to be said. As is well known, there are many grades of color-blindness. We do not know whether these grades are due to individual, non-genetic, variations — assuming it to be due to one gene; or whether there are several genes that differ in the degree to which they produce the defect. We know of a good many cases in other animals where there are several mutations of the same gene. For instance, in Drosophila there is a series of ten such multiple allelomorphs for eye colors that range from pure white to deep wine-red.

Morgan 1925 fig69.jpg

Fig. 69. Diagram to show the inheritance of color-blindness in man. The eye that can distinguish red from green is here half black, half barred, while the color-blind eye is stippled. A colorblind man mates with a normal woman. The sons and daughters are normal. Two individuals of such parentage give three normals to one color-blind individual in F2. The color-blind individual is always a male.

Morgan 1925 fig70.jpg

Fig. 70. The designations as in fig. 69. Here a color-blind woman mates with a normal male. All of her sons are color-blind, her daughters have normal vision (but carry a factor for colorblindness). In the F2 generation, half the daughters and half the sons have normal eyes and half are color-blind.

There is still another possible interpretation of the different kinds of color-blindness — one which a priori would seem to be the most probable namely, that the differences are due to other modifying genes that affect the extent to which the character develops.

Morgan 1925 fig71.jpg

Fig. 71. Two pedigrees for color blindness. (After Lenz.)

Blue eyes in man behaves as a recessive to brown eyes. Two blue-eyed parents have only blue-eyed children (fig. 72). Pure brown-eyed individuals have only brown-eyed children (fig. 72), but a brown-eyed individual, one of whose parents had blue eyes, married to a blue-eyed individual has both blue- and brown-eyed children in equal numbers according to Mendelian expectation. In recent years a few cases have been recorded where two blue-eyed parents have had some brown-eyed children, and this has furnished the opponents of Mendelian inheritance with an argument against the general application of Mendel's theory. Such cases are, however, only an argument against an overstatement of that theory as always applying to apparently blue-eyed individuals. It is known that occasionally blue-eyed individuals have only a speck of brown pigment in their eyes. They may then produce some brown-eyed children. In other words, the hybrid brown eye-color is variable in extent, and at one extreme shows almost no brown color or possibly none at all, and yet is genetically brown-eyed. That this is the true explanation is shown by the pedigree of these genetic browns, for, so far as recorded they have had at least one brown-eyed parent. In other words, in extreme and exceptional cases possibly due to weakness or disease the brown eye-color may not develop in an individual that is genetically a brown hybrid. This failure of the somatic character does not affect the brown-producing gene, for, such individuals behave in inheritance as hybrid brown eyed individuals.

Morgan 1925 fig72.jpg

Fig. 72. Diagram to illustrate the inheritance of blue eyes (to the left), and of brown eyes (to the right).

Albinism in man has been known for a long time and the earlier records of white Indians seem, in the light of recent discoveries, not to be mythical. In all races, including negros, albinos are known. It has been estimated that this occurs once in 5,000 to once in 30,000 individuals. It is possible that there is more than a single kind of albino due to mutation in different genes or to allelomorphic mutations that give different degrees of deficiency of pigment.

Albinism is a recessive character. A few cases are on record where two albinos have had only albino children. In true albinos the brown pigment is absent from the skin, hair, and iris. Its absence in the iris gives the eye a pink color due to the blood in the back of the eye, showing through the semitransparent iris.

A defect in vision known as stationary nightblindness has been shown by Nettleship to be a dominant Mendelian trait (fig. 73) . In one locality (near Montpellier in France) it has been traced to the year 1637 and hence has been handed down for about two hundred and fifty years. There are other types of night-blindness that have a different inheritance not entirely made out.

Morgan 1925 fig73.jpg

Fig. 73. Diagram to show the inheritance of night-blindness. (After Nettleship.)

Haemophilia in man has been shown to be transmitted as a sex-linked character (fig. 74) . The blood of affected individuals fails to coagulate quickly when exposed to air, hence there is danger of the individual bleeding to death. Several pedigrees have been made out. It is a recessive character whose gene is carried by the A^-chromosome. It appears in any male whose single A^-chromosome carries the gene for haemophilia. Its relative infrequency in women is explained on the grounds that it can appear in them only when the father and mother both possess the character or when the mother herself has had a haemophilic father — in other words when both X chromosomes carry the gene.

The Four Blood Groups and their Inheritance

One of the most remarkable cases of heredity in man is found in the so-called blood groups. As first shown by Von Dungern and Hirschfeld in 1910 the inheritance of the four blood groups conforms to

Morgan 1925 fig74.jpg

Fig. 74. Diagram to illustrate the inheritance of bleeding or haemophilia. (After Bulloch and Fildes.)

Mendel's laws. So consistent is this relation that, as Ottenberg pointed out in 1921, the evidence might be used in certain cases to determine the parentage of the child. The presence of two pairs of factors will account for the results. Thus if one pair of genes be represented by A and a and the other pair by B and h, and if an individual with the genetic constitution AaBh be mated to another individual of like constitution (AaBb), then each will contain four kinds of germ cells, viz., AB, Ah, Ba, and ah. The sixteen possible combinations formed, if any sperm may fertilize any egg^ are shown in figure 75.

These sixteen individuals fall into four groups according to whether they have both A and B, or


Mating of blood group AaBb to same AaBb only A, or only B, or neither A nor B (i.e., ah) in the proportion of 9AB:3A:3B:lah. These four genetic classes correspond to the four recognized blood types IV, II, III, I, as indicated in the diagram.


Morgan 1925 fig75.jpg

Fig. 75. Diagram illustrating the sixteen classes of individuals when two members of the blood group AaBa mate. There are four classes of individuals produced, indicated by the circles, lines, squares, and absence of A and B.


Now these sixteen kinds of individuals are found in all populations, so far studied, although in somewhat different proportions in different "races."

It is very simple to tell what the kinds of genetic offspring will be when any one of these sixteen individuals marries any other one. These possibilities are summarized in the following statement taken from Ottenberg :

Morgan 1925 chapter13-Ottenberg blood groups.jpg

Two actual pedigrees, one of them carried through three generations, will serve to illustrate particular cases (fig. 76).


From a knowledge of the blood group to which the child belongs it is possible to predict to what groups its parents may have belonged, and in certain cases it is possible to state that an individual of a certain group could not have been the parent of a particular child.


Morgan 1925 fig76.jpg

Fig. 76. Two pedigrees showing the inheritance of two blood groups. See text.


In the transfusion of blood from one individual to another, that is sometimes necessary, it is essential that the blood corpuscles of the donor are not agglutinated by the serum of the recipient. Thus it is a matter of great importance to select a donor that does not bring about such a catastrophe. The simple rules are that individuals belonging to the same blood group (I, II, III, or IV) do not agglutinate each other's blood, but the blood corpuscles of an individual represented by AA or A a will be precipitated if the donor contains the agglutinin represented by aa, and similarly the blood corpuscles of an individual represented by BB or Bh will be precipitated if the donor contains the agglutinin represented by bb. Inspection of the diagram will show that group II (with serum bb) precipitates III and IV, and group III (with serum aa) precipitates II and IV. Further, the serum of group I (aa bb) precipitates all of the other groups; while the serum of group IV precipitates none of the others.

Inheritance of Other Traits

There are numerous other physical characters of man that are evidently inherited but where the numher of factors involved is uncertain or entirely unknown. Some of these characters are present in all races. Others to some extent are racial characters. Thus height in man is a very variable character. It is obyiously a complex of several or many elements little understood. Differences in length of legs, or of body, or of neck, and of different combinations of these may be present. Height is a growth phenomenon depending amongst other things on the time at which the growth of the bones, especially the long bones of the leg, stops, and this in turn is, to some extent at least, connected with the time of sexual maturity, which depends again on the time of functioning or the amount of secretion produced by some of the glands of internal secretion (testes or ovary, thyroid, pituitary, etc.). These inter-relations have made the study of growth very difficult especially when the determination of the rate of growth or its cessation is connected with internal organs that are seldom seen or measured. Nevertheless there is nothing in these complications that precludes the possibility that the ultimate source of the variability is due to genetic factors.


There is little accurate information at present as to the number of factor differences that are involved in the inheritance of hair color in man. The changes in color that take place during the life of the individual renders its study difficult. An individual child may start with flaxen hair, later have brown hair, and in old age become white-headed. Three generations living at the same time may show these differences. A complete record would then have to extend over several years; for, hearsay evidence as to the color of the hair of the grandparent when in middle age may be inaccurate and the future color of the hair of a child would be largely guessing. Data collected by the questionnaire method, that has been used in the study of the inheritance of hair color, can not be relied upon without some more definite standard than popular designations of shades of color.

The Inheritance of Mental Traits

Man's success as a social animal depends as much on his mental qualities as on his physical characteristics. No one will deny, I suppose, that men behave in different ways, but who can say how far differences in human behavior depend on the physique of the individual, how far on his early experiences and training, and how far on differences in his sense organs and central nervous system? Until some of these questions are better understood it is impossible to know how far observed differences are innate and how far acquired.


Here again, as in the case of man's physical defects, there are a few extremely abnormal conditions where the evidence indicates that something is inherited, but even here there is much that is obscure. The case most often quoted is feeble-mindedness that has been said to be inherited as a Mendelian recessive, but until some more satisfactory definition can be given as to where feeble-mindedness begins and ends, and until it has been determined how many and what internal physical defects may produce a general condition of this sort, and until it has been determined to what extent feeble-mindedness is due to syphilis, it is extravagant to pretend to claim that there is a single Mendelian factor for this condition.


Family pedigrees in which an unusual number of individuals below par are present undoubtedly give the impression that something is inherited, but until all the social conditions surrounding the childhood of the individual are examined and given proper weight, serious doubts will arise as to what form of inheritance is producing the results. It is quite probable that there are extraneous factors involved in such pedigrees.


There is no a priori objection to the assumption that different sense organs and different brains react as differently as do other organs of the l)ody. Those that react below some selected standard might be called feeble-minded; but there are no grounds for assuming that the results are due to one particular defect in the nervous system, and in fact a critical study of the cases shows that they are probably not all due to a single factor difference or even to the same ones. The pedigrees that have been published showing a long history of social misconduct, crime, alcoholism, debauchery, and venereal diseases are open to the same criticism from a genetic point of view^; for it is obvious that these groups of individuals have lived under demoralizing social conditions that might swamp a family of average persons. It is not surprising that, once begun from whatever cause, the effects may be to a large extent communicated rather than mherited.


It is quite possible of course that an inherited defective dominant character might furnish the starting jDoint for these histories, but that the subsequent events are all due to "bad blood" or "defective germplasm" remains to be shown.


"Insanity" is another "psychological trait" that is said to be inherited and the numerous pedigrees that have been collected showing that certain types of insanity occur more frequently in certain families than in others seem to furnish evidence in support of such a claim. This is particularly the case in Huntington's chorea (fig. 77) a type of insanity often leading ultimately to suicide, that does not appear as a rule until middle life or later. Since it appears to be dominant, its history is more easily followed than in most other cases where the dominance or recessiveness is in question. Huntington's chorea has been traced in a limited group of individuals. The background of its expression appears to be connected in some way with the sex organs but what this connection may be is unknown, for it appears in both sexes which makes it difficult to account for the disturbance on the basis of a sex endocrine.

Morgan 1925 fig77.jpg

Fig. 77. Chart showing; the inheritance of Huntington's chorea. (After Davenport.)


At best one can say, perhaps, that in certain strains and perhaps under certain conditions mental disorders appear, but so long as neither the physiological background of insanity, or the external agents, that are contributory, are known, its genetic relations must remain obscure.


If these "best cases" are so far from being established on a scientific footing, it is not particularly profitable to discuss the many claims that have been set up for other mental traits, even though it must be conceded that defective characteristics might be the ones, judging by analogy with mutant physical defects, that would be more likely to furnish evidence of Mendelian inheritance than the less extreme differences that distinguish "normal" individuals. The important point, however, to be urged is that the "mental traits" in man are those that are most often the product of the environment which obscures to a large extent their inheritance, or at least makes very difficult their study.


While the inheritance of disorders relative to human behavior are of importance to the pathologist and to the penologist, the inheritance of individual differences that fall within what would be called the "normal" is more important from the point of view of human evolution. Here also we are on very dubious grounds when we discuss the inheritance of individual mental peculiarities, and in still greater danger of error if we attempt to discriminate between racial complexes. The similarity in behavior or in "temperament" or mental qualities of identical twins might be expected to furnish important information as to how much is acquired and how much inherited. The very close physical similarity of twins of this kind might make such material favorable for study. There are, however, even here two serious drawbacks that complicate the results. In the first place unless the twins had been separated in very early childhood it would be difficult to decide how much is due to similarity of nature and how much to nurture. A comparison with other children in the same family may be helpful but is not decisive, for the experience of each child from successive births is affected by older and younger children in the family. In the second place the so-called standard tests may measure training rather than constitutional factors. Until these difficulties can be overcome, the many anecdotes of the close similarity in temperaments, or abilities, of identical twins do not supply the needed evidence.


We can, by artificial selection, eliminate structural defects from a race of animals or plants and by proper breeding make the race more uniform and maintain it at or near a chosen standard. Since we have many good reasons to think that man's physical inheritance conforms to the same principles that apply to other animals, it follows that by elimination and suitable mating man too could be standardized. How far one might have to go in order to carry out this reformation is a matter of opinion. If too strenuous standards were set up the human race might be exterminated before the reformation began. Genetic reformers and racial propagandists do little more than recommend cutting off a few of the most defective individuals. But it is not so much the physically defective that appeal to their sympathies as the "morally" deficient and this is supposed to apply to mental traits rather than to physical characters. Ruthless genetic (?) reform here might seem too drastic and might be retroactive if pressed too far. Social reforms might, perhaps, more quickly and efficiently get at the root of a part of the trouble, and until we know how much the environment is responsible for, I am inclined to think that the student of human heredity will do well to recommend more enlightenment on the social causes of deficiencies rather than more elimination in the present deplorable state of our ignorance as to the causes of mental differences.

Lest it appear from what has been said that I have too little faith in the importance of breeding for mental superiority I should like to add that I am inclined to think that there are considerable individual differences in man that are probably strictly genetic, even though I insist that at present there is for this no real scientific evidence of the kind that we are familiar with in other animals and in plants. I will even venture to go so far as to suppose that the average of the human race might be improved by eliminating a few of the extreme disorders, however they may have arisen. In fact, this is attempted at present on a somewhat extensive scale by the segregation into asylums of the insane and feebleminded. I should hesitate to recommend the incarceration of all their relatives if the character is suspected of being recessive, or of their children if a dominant. After all, these segregations are based on humanitarian principles, or for our protection rather than for genetic reasons. How long and how extensively this casual isolation of adults would have to go on to produce any considerable decrease in defectives, no informed person would, I should think, be willing to state.


Least of all should we feel any assurance in deciding genetic superiority or inferiority as applied to whole races, by which is meant not races in a biological sense but social or political groups bound together by physical conditions, by religious sentiments, or by political organizations. The latter have their roots in the past and are acquired by each new generation as a result of imitation and training. If it is unjust "to condemn a whole people’p meaning thereby a political group, how much more hazardous is it, as some sensational writers have not hesitated to do, to pass judgment as to the relative genetic inferiority or superiority of different races.


If within each human social group the geneticist finds it impossible to discover, with any reasonable certainty, the genetic basis of behavior, the problems must seem extraordinarily difficult when groups are contrasted with each other where the differences are obviously connected not only with material advantages and disadvantages resulting from location, climate, soil, and mineral wealth, but with traditions, customs, religions, taboos, conventions, and prejudices. A little goodwill might seem more fitting in treating these complicated questions than the attitude adopted by some of the modern race propagandists.


  1. Assuming the parent is heterozygous for the character.



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