Heredity and Sex (1913) 5

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A personal message from Dr Mark Hill (May 2020)  
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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.

Heredity and Sex (1913): 1 Evolution of Sex | 2 Mechanism of Sex-Determination | 3 Mendelian Principles of Heredity and Bearing on Sex | 4 Secondary Sexual Characters Relation to Darwin's Theory of Sexual Selection | 5 Effects of Castration, Transplantation on Secondary Sexual Characters | 6 Gynandromorphism, Hermaphroditism, Parthenogenesis, and Sex | 7 Fertility | 8 Special Cases of Sex-Inheritance | Bibliography
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) 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, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)


Chapter V The Effects of Castration and of Transplantation on the Secondary Sexual Characters

In several of the preceding chapters I have spoken in some detail of sex-linked inheritance. In sex-Unked inheritance we deal with a class of characters that are transmitted to one sex alone in certain combinations, and have for this reason often been called sex-limited characters; but these same characters can be transferred by other combinations, as we have seen, to the other sex, and are therefore not sex-limited.

In contrast to these characters secondary sexual characters appear in one sex only and are not transferable to the other sex without an operation. For instance, the horns of the stag and the colors and structures of certain male birds are in nature associated with one sex alone.

It has long been recognized in mammals and birds that there is a close connection between sexual maturity and the full development of the secondary sexual characters. This relation suggests some intimate correlation between the two. It has been shown, in fact, in some mammals at least, that the development of the secondary sexual characters does not take place, or that they develop imperfectly, if the sex glands are removed. It may appear, therefore, that we are dealing here with a purely physiological process, and that the development of these structures and colors is a byproduct of sex itself, and calls for no further explanation.

But the question cannot be so hastily dismissed. This can best be shown by taking up at once the material at hand.

Operations on Mammals

In the deer, the facts are very simple. If the very young male is castrated before the knobs of the antlers have appeared, the antlers never develop.



Fig. 67. — Merino; male (horned) and female (hornless).

If the operation is performed at the time when the antlers have already begun to develop, incomplete development takes place. The antlers remain covered by the velvet and are never thrown off. They are called peruke antlers. If the adult stag is castrated when the horns are fully developed, they are precociously dropped, and are replaced, if at all, by imperfect antlers, and these are never renewed.

These facts make it clear that there is an intimate relation between the orderly sequence of development of the horns in the deer and the presence of the male sexual glands.

In the case of sheep, the evidence is more explicit. Here we have carefully planned experiments in which both sexes have been studied ; and there are breeding experiments also, in which the heredity of horns has been examined.




Fig. 68. — Dorset; male (horned) and female (horned).


In some breeds of sheep, as in the Merinos and Herd wicks, horns are present in the males, absent in the females (Fig. 67). In other breeds of sheep, as in Dorsets, both males and females have horns (Fig. 68). In still other breeds both sexes lack horns, as in some of the fat-tailed sheep of Africa and Asia (Fig. 69).

Marshall has made experiments with Herdwicks — a race of sheep in which the rams have large, coiled horns and the ewes are hornless. Three young rams (3, 4, and 5 months old) were castrated. The horns had begun to grow (3, 4^^, and 6 inches long) at the time of operating. They ceased to grow after the operation.

A similar operation was also carried out on females. Three Herdwick ewe lambs (about 3 months old) were operated upon. After ovariotomy, the animals were kept for 17 months, but no horns appeared, although in one, small scurs developed, in the other two scarcely even these. It is clear that the removal of the ovaries does not lead to the development of horns like those in the male.

Now, the interpretation of this case can be made only when taken in connection with experiments in heredity. There is a crucial experiment that bears on this question. Arkell found when a Merino ewe (a race with horned males and hornless females) was bred to a ram of a hornless breed, that the sons had horns. In this case the factor for horns must have come from the hornless mother, while the development of the horns was made possible by the presence of the male glands. It is evident therefore in the castration experiment that a factor for horns is inherited by both sexes, but in order that the horns may develop fully, the male glands must be present and functional.

In the Dorset, both sexes are horned, the horns of the females are lighter and smaller than the horns of the ram (Fig. 68). In the castrated males the horns are like those of the females. In this case we must suppose that the hereditary factor for horns suffices to carry them to the point in development reached by the females. To carry them further the presence of the sex glands of the male is necessary.

In the case of the hornless breeds I do not know of any evidence from castration or ovariotomy. We may suppose, either that the factor for horns is absent ; or, if present, some inhibitory factor must bring about suppression of the horns. The former assumption seems more probable, for, as I shall point out, certain experiments in heredity indicate that no inhibitor is present in hornless breeds.

The series is completed by cases like that of the eland and the reindeer. Both males and females have well-developed horns. In this case the hereditary factors suffice in themselves for the complete development of horns, for even after castration the horns develop.


Fig. 69. — Fat-tailed hornless sheep (Ovis aries steatopyga persicci).



We have anticipated to some extent the conclusions arrived at by breeding experiments in these races of sheep. The best-known case is that of Wood, who crossed horned Dorsets and hornless Suffolks. As shown in the picture (Fig. 70) the sons had horns — the daughters lacked them. When these are inbred, their offspring are of four kinds, horned males, hornless males, horned females, hornless females.

It seems probable that these four classes appear in the following proportions :

Horned S Hornless S Horned $ Hornless ? 3 113

The explanation that Bateson and Punnett offer for this case is as follows : The germ-cells of the horned race








Fig. 70. — 1, Suffolk (ram), hornless in both sexes; 2, Dorset (ewe), horned in both sexes ; 3, Fi ram, horned ; 4, Fi ewe, hornless ; 5-8, the four types of F2 ; 5 and 6 are rams, 7 and 8 are ewes. The hornless rams are pure for absence of horns, and the horned ewes are pure for the presence of horns. Figs. 5 and 6 represent lambs. (Bateson, after Wood.)


(both male and female) carry the factor for horns (H) ; the germ-cells of the hornless race lack the factor for horns (h). The female is assumed to be homozygous for the sex factor, i.e. two sex chromosomes (X) are present ; while the male has only one sex chromosome carried by the female-producing sperm. The analysis is then as follows: One "dose" of horns (H) in the male produces horns, but two doses are necessary for the female.

Hornless ? hX — hX

Horned S H X — H


F,


H X h X hornless ? H h X horned 6


Gametes f Eggs H X — hX

ofFi l^permH X — hx — H — h


F2 Females

H X H X horned H X h X hornless h X H X hornless h X h X hornless


F2 Males

H H X horned H h X horned h H X horned h h X hornless


As pointed out by Punnett a test of the correctness of this interpretation is found by breeding the Fi hornless female to a hornless male (of a hornless breed). It is assumed that such a female carries the factors for horns in a heterozygous condition ; if so, then half of her sons should have horns, as the following analysis shows :

Fy Hornless 9 H X — hx Hornless 6 h x — h

h X H X hornless 9

h X h X hornless ?

h H X horned S

h h X hornless 6





Fig. 71. — Upper figure normal male guinea pig (from below), to show mammary glands. Lower figure, a feminized male ; i.e. castrated when three weeks old and pieces of ovaries transplanted beneath the skin, at Ov.


The actual result conforms to the expectation. The results of both of the experiments are consistent with the view that one factor for horns in the male produces horns, which we may attribute to the combined action of the inherited factor and a secretion from the testes which reenforces the action of the latter. This, however, should be tested by castrating the Fi males. In the females, one factor for horns fails to produce horns, while two factors for horns cause their development.

Aside from some of the domesticated animals (horses, cattle, dogs, cats, pigs), the only other mammals on which critical experiments have been made — if we exclude man — are the rat and the guinea pig. The next case is unique in that the ovary was transplanted to a male.

Steinach removed the sex glands from the male guinea pig and rat and transplanted into the same animals the ovaries of the female, which established themselves. Their presence brought about remarkable effects on the castrated male. The mammary glands, that are in a rudimentary condition in the male, become greatly enlarged (Fig. 71). In the rat the hair assumes the texture of that of the female. The skeleton is also more like that of the female than the male. The size of the feminized rats and guinea pigs is less than that of normal (or of castrated) males and like that of the female (Fig. 72). Finally, in their sexual behavior, the feminized rats were more like females than like males. These cases are important because they are the only ones in which successful transplanting of the ovary into a male has been accomplished in vertebrates.


Fig. 72. — Two upper figures, normal male guinea pig to left, M, and his brother, F, to right — a feminized male. Two middle and two lower figures, a normal male at M, and his feminized brother, F. (After Steinach.)


Operations on Birds

In striking contrast to these results with mammals are those with birds, where in recent years we have gained some definite information concerning the development of secondary sexual characters.

I am fortunate in being able to refer to several cases — the most successful on record — carried out by my friend, H. D. Goodale, at the Carnegie Laboratory at Cold Spring Harbor. One case" is that of a female Mallard duck from which the ovary was completely removed when she was a very young bird. Figure 16 illustrates the striking difference between the normal male and the female Mallard. In the spayed female the plumage is like that of the male.

Darwin records a case in which a female duck in her old age assumed the characteristics of a male, and similar cases are recorded for pheasants and fowls.

Goodale also removed the ovary from very young chicks. He found that the female developed the secondary sexual plumage of the cock.

How shall we interpret these cases ? It is clear that the female has the potentiality of producing the full plumage of the male, but she does not do so as long as the ovary is present. The ovary must therefore be supposed to prevent, or inhibit, the development of secondary sexual characters that appear therefore only in the male.

The converse operation — the removal of the male glands from the male — is an operation that is very common among poultry men. The birds grow larger and fatter. They are known as capons. In this case the male assumes his full normal plumage with all of his secondary male sexual characters. It is said that the comb and wattles and to some extent the spurs are less developed in the capon than in the normal male. But aside from this it is quite certain that the development of the secondary sexual plumage in the male is largely independent of the presence of the sex glands.




Fig. 73. — Male and female Seabright. Note short neck feathers and incomplete tail cover in male. In the Seabright cock the sickle feathers on back at base of tail are like those of the hen. (After "Reliable PoultryJournal.")


The method of inheritance of the secondary sexual characters in birds has been little studied. Davenport has reported one case, But I am not sure of his interpretation.^ I have begun to study the question by using Seabright bantams, in which the male lacks some

1 Because it is not evident whether the secondary sexual characters as such are involved or only certain general features of coloration.


144 HEREDITY AND SEX

of the secondary sexual characters of the domestic races, notably the saddle feathers, as shown in Fig. 73.

When a female Seabright was mated to a blackbreasted game male the sons had the secondary sexual plumage of the father.

In the second generation, however, some of the males showed imperfect development of the sickle feathers to various degrees — some to the extent shown by the Seabright. It appears that the female transmits the features peculiar to the male of this race.


Seabright


?


sF


-s


Game 6



S S




SsF Normal ?


F,


1


Ss Normal 6


Gametes [


SF

- sF — S — s Eggs


oiF, 1



S — s Sperm




SSF]





SsF sSF


Normal ?




ssF.



F^



SS Normal c? Ss Normal S sS Normal S




ss


Seabright 6


In conclusion, then, in mammals the secondary sexual characters owe their development to the testes. The testes add something to the common inheritance. But in birds the ovary takes something away.


Operations on Amphibia

The male triton develops each year a peculiar fin or comb on the back and tail. Bresca has found that after castration the comb does not develop. If present at the time of castration, the comb is arrested, but only after several months. Certain color marks peculiar to the male are not lost after castration. If the comb is removed in normal males, it regenerates, but less perfectly in castrated males. If a piece of the dorsal fin of the female is transplanted to a normal male in normal position, it may later produce the comb under the influence of the testes.

In the frog there appears at the breeding season a thickening of the thumb. Castrated males do not produce this thickening.

If it is present in a male at the time of castration it is thrown off, according to Nussbaum, but according to Smith and Shuster its further progress only is arrested. According to Nussbaum and Meisenheimer injection of pieces of testes beneath the skin of a castrated male cause the thumb development to take place, or to continue, but Smith and Shuster question this conclusion.

Such are the remarkable relations that these experiments have brought to Hght. How, we may ask, do the sex glands produce their ^effect, in the one case to add something, in the other to suppress something?

It has often been suggested these glands produce their effects through the nervous system by means of the nerves to or from the reproductive organs. This has been disproved in several cases by cutting the nerves and isolating the glands. The results are the same as when they are left intact.

This brings us to one of the most interesting chapters of modern physiology, the production and influence of Internal Secretions.

Internal Secretions

It has become more and more probable that the effects in question are largely brought about by internal secretions of the reproductive organs. These secretions are now called '^hormones" or exciters.!' They are produced not only by glands that have ducts or outlets, but by many, perhaps by all, organs of the body. Some of these secretions have been shown to have very remarkable effects. A few instances may be mentioned by way of example.

The pituitary body produces a substance that has an important influence on growth. If the pituitary body becomes destroyed in man, a condition called gigantism appears. The bones, especially of the hands and feet and jaws, become enlarged. The disease runs a short course, and leads finally to a fatal issue.

The thyroid and parathyroid bodies play an important role in the economy of the human body through their internal secretions. Removal leads to death. A diseased condition of the glands is associated with at least six serious diseases, amongst them cretinism.

The thymus secretion is in some way connected with the reproductive organs. Vincent suggests that the thymus ministers to certain needs of the body before the reproductive organs are fully developed."


Extirpation of the adrenal bodies, another ductless gland, leads to death. Injury to these bodies causes Addison's disease.

Finally, the reproductive glands themselves produce internal secretions. In the case of the male mammal it has been shown with great probability that it is the supporting tissues of the glands, and not the germ-cells, that produce the secretion. Likewise, in the case of the ovary, it appears that the follicle cells of the corpus luteum give rise to an important internal secretion. If the sac-like glands are removed, the embryo fails to become attached to the wall of the uterus of the mother. If the ovary itself is removed from a young animal, before corpora lutea are formed, the uterus remains in an infantile condition.

From a zoological point of view the recent experiments of Gudernatsch are important. He fed young frog tadpoles with fresh thyroid glands. " They began very soon to change into frogs, but ceased to grow in size. The tadpoles might begin their metamorphosis in a few days after the first application of the thyroid, and weeks before the control animals did so."

In contrast to these effects Gudernatsch found that tadpoles fed on thymus grew rapidly and postponed metamorphosis. They might even, in fact, fail to change into frogs and remain permanently in the tadpole condition. If thyroid extracts produce dwarfs ; thymus extracts make giant tadpoles that never become adults.

These examples will suffice to show some of the important effects on growth that these internal secretions may bring about.


Operations on Insects

The Insects constitute the third great group in which secondary sexual characters are common.

The first operations on the reproductive organs were carried out by Oudemans on the gipsy moth, Ocneria (Porthetria) dispar. The male and female are strikingly different. Oudemans removed the testes from



Fig. 74. — Ovaries of Lymantria {Porthetria) dispar transplanted to male. They have established connection with the sperm ducts. (After Kopec.)

young caterpillars and found no change in the color, or size, of the male. He also removed the ovaries from young caterpillars, and again found no effect in the female. The same experiments were later carried out on a large scale by Meisenheimer, who obtained similar results. Meisenheimer went further, however, and performed another operation of great interest. He removed the male glands from a male and implanted in their place the ovary of a female, while it was still in a very immature condition. The caterpillar underwent its usual growth, changed to a chrysalid, and then to a moth. The moth showed the characters of the male. The presence of the ovary had produced no effect whatever on the body character of the individual. When this individual was dissected, Meisenheimer found that the ovary had completely developed. It contained mature eggs, and the ovary had often established connection with the outlets of the male organs that had been left behind, as seen in Fig. 74, taken from Kopec's description.



Fig. 75. — Testes of Lymantria (Porthetria) dispar transplanted to female. They have connected with the oviducts. (After Kopec.)



The converse experiment was also made. The ovaries were removed from young caterpillars, and in their place were implanted the male sex glands from a young male caterpillar. Again no effects were produced on the moth, which showed the characteristic female size and color. On dissection the testes were also found to have grown to full size and to have produced spermatozoa (Fig. 75).

These remarkable results, confirmed by Kopec, show that in these insects the essential organs of reproduction have no influence on the secondary sexual characters of the individual. They show furthermore that the male generative organs will develop as well in the female ^s in the body of the male itself, and vice versa.

It is evident, then, in insects (there is a similar, but less complete., series of experiments on the cricket), that the heredity of the secondary sexual characters can be studied quite apart from the influence of the sex glands. How, then, are they inherited so that they appear in one sex and not in the other sex? Within the last two or three years the inheritance of the secondary sexual differences in insects has been studied.




Fig. 76. — Papilio Memnon. 1, male; 2, 3, 4, three types of females. (After Meijere.)


First, there is the case of the clover butterfly, Colias philodice, that Gerould has worked out, where there are two types of females and one kind of male (Fig. 66).

Without giving the analysis of this case I may say that the results can be explained on a Mendelian basis. The peculiar feature of Gerould's explanation is that two doses of the yellow-producing determiner in the female give yellow color — one dose gives white. In the male, on the other hand, one dose of yellow gives yellow.

The second case is that of Papilio memnon, worked out by de Meijere from the experiments of Jacobson. There is one male type and three female types, Fig. 76. De Meijere accounts for the results of matings in this species recorded by Jacobson on the assumption of three factors, one for each type of female. The three factors are treated as allelomorphs, and therefore only two of them can be present in any one individual, and since they are allelomorphs they will pass into different gametes. The order of dominance is Achates, Agenor, Laomedon. The male carries these same factors, but they are not effective in him. Baur accounts for the results in a somewhat different way, but involving ordinary Mendelian conceptions.

An interesting case is that reported by Foot and Strobell. They crossed a female of a bug, Euschistus variolarius, the male of which has a black spot on the end of the body (the female lacking the spot), with a male of Euschistus servus that lacks the spot both in the males and the females (Fig. 77). The daughters had no spot ; the sons had a faint spot, less developed than in variolarius. When these (Fi) offspring were inbred, they obtained 249 females without a spot.


107 males with a spot (developed to different degrees), and 84 males without a spot. The authors give no explanation of their results — but they use the re suits to discredit some of the explanations, that rest on the assumption that the chromosomes are the chief factors in Mendelian heredity. I venture, nevertheless, to suggest the explanation shown on the accompanying diagram (Fig. 78). The analysis rests on the assumption that neither one nor two doses of S in the female is able to produce a spot, while in the male one dose of S suffices.



FiG. 77. — To left, in 1, is male of Euschistus variolarius, to right male of E. servus. 2 and 3 show eight F2 males; 4 shows seven F2 males from another mating.



Fig. 78. — Diagram showing a possible interpretation of the heredity of spot of male when E. servus is crossed with E. variolarius. S = spot; s = no spot.

It is very important to understand just what is meant by this ; for otherwise it may seem only like a restatement of the facts. In the F2 female with the formula SX SX, i.e. two doses of the S factor, no spot is assumed to appear (nor in the hybrid female SXsX). At first sight it seems that a female having the formula SX SX is only double the male with SXs, especially if small s is interpreted to mean absence of spots. But this view, in fact, involves a misconception of what the factorial hypothesis is intended to mean.


To make this clearer, I have written out the case more fully :

X A BC S X AB CS 9 X ABC S A BCs 6

In this, as in all such Mendelian formulae, the result (or character) that a factor produces depends on its relations to other things in the cell (here ABC). We are dealing, then, not with the relation of X to aS alone, but this relation in turn depends on the proportion of both X and S to A B C. It is clear, if this is admitted, that the two formulae above — the one for the male and the other for the female — are neither identical nor multiples.

It will be noted that in only one of these attempts to explain in insects the heredity of the secondary sexual characters have the factors for the characters been assumed to be caused by the sex chromosomes. If one accepts the chromosome basis for heredity, these results may be explained on the assumption that the factors lie in other chromosomes than the sex chromosomes.

In the next case, however, that I shall bring forward the factors must be assumed to be in the sex chromosomes themselves.

The mutant of drosophila with eosin eyes that arose in my cultures is the case in question. The female has darker eyes than the male. The experimental evidence shows that the factor for eosin is carried by the sex chromosomes. In the female it is present, therefore, in duplex, or, as we say, in two doses; in the male in one dose.


The difference in color can be shown, in fact, to be due to this quantitative relation. If, for instance, an eosin female is mated to a white-eyed male, her daughters have light eyes exactly like those of the eosin male. The white-eyed fly lacks the eosin factor in his sex chromosomes (as suitable matings show), hence the hybrid female has but one dose of eosin, and in consequence her eye color becomes the same as the male.

In this case a sex-linked character is also a secondary sexual character because it is one of the rather unusual cases in which a factor in two doses gives a stronger color than it does in one dose.

Parasitic Castration Of Crustacea

Let us turn now to a group in which nature performs an interesting operation.

Giard first discovered that when certain male crabs are parasitized by another crustacean, sacculina (a cirriped or barnacle) , they develop the secondary sexual characters of the female. Geoffrey Smith has confirmed these results and carried them further in certain respects. Smith finds that the spider crab, Inachus mauritanicus, is frequently infected by Sacculina neglecta (Fig. 79). The parasite attaches itself to the crab and sends root-like outgrowths into its future host. These roots grow like a tumor, and send ramifications to all parts of the body of the crab.

The chief effect of the parasite is to cause complete or partial atrophy of the reproductive organs of the crab, and also to change the secondary sexual characters. Smith says that of 1000 crabs infected by sacculina, 70% of both males and females showed alterations in their secondary sexual characters.

As a control, 5000 individuals not infected were examined, only one was unusual, and this one was a hermaphrodite (or else a crab recovered from its parasite).




Fig. 79. — A male of Inachiis mauriLinicus (upper left hand). Female of Inachus scorpi (lower left hand). Male of Inachus mauritanicus carrying on its abdomen two specimens of Danalia curvata and a small Sacculina neglecta (upper right hand). Male of Inachus ynauritanicus with a Sacculina neglecta on it (lower right hand). The abdomen and cheliB of the host are intermediate in character between those of an ordinary male and female. (After Geoffrey Smith.)


As the figures (Fig. 80) show, the adult male has large claws ; the female, small ones. He has a narrow abdomen; she has a broad one. In the male there is a pair of stylets on the first abdominal ring (and a pair of greatly reduced appendages behind them). The adult female has four biramous abdominal appendages with hairs to carry the eggs.



Fig. 80. — 1, adult normal male; 2, under side of abdomen of normal


The infected males show every degree of modification towards the female type." The legs are small, the abdomen broad, the stylets reduced, and the typical biramous appendages with hairs appear.

When the female crab is infected she does not change 'Howard" the male type, although the ovary may be destroyed. The only external change is that the abdominal appendage may be reduced.

In a hermit crab, Eupagurus rneticulosus, infected by Peltogaster curvatus, similar results have been obtained. The infected male assumed the ordinary sexual characters of the female, but the females showed no change towards the male.

In these cases it seems probable that the testes of the male suppress the development of the secondary sexual characters that appear ordinarily only in the females. The case is the reverse of that of the birds and different again from that of the mammals.

In birds and mammals the secondary sexual characters are in many cases directly dependent on the internal secretions of the sex glands. These secretions are carried alike to all parts of the body, hence the absence of bilateral gynandromorphs in these groups.

adult male; 3, male infected with sacculina, showing reduction of chela and slight broadening of abdomen; 4, 5, showing attenuated copulatory styles and slight hollowing out of abdomen; 6, under side of abdomen of a similar male specimen, showing reduction of copulatory styles and presence of asymmetrically placed swimmerets characteristic of female; 7, infected male which has assumed complete female appearance; 8, under side of abdomen of 7, showing reduced copulatory styles and swimmerets; 9, under side of abdomen of similar male specimen with well-developed copulatory styles and swimmerets; 10, adult female, normal; 11, under side of abdomen of 10, showing swimmerets and trough-shaped abdomen; 12, under side of abdomen of infected female, showing reduction of swimmerets; 13, immature female showing small flat abdomen; 14, under side of abdomen of 13, showing flat surface and rod-like swimmerets. (After Geoffrey Smith.)


Conclusions

In conclusion it is evident that the secondary sexual characters in four great groups, viz. mammals, birds, Crustacea, and insects, are not on the same footing. Their development depends on a different relation to the reproductive organs in three of the groups, and is independent of the reproductive organs in the fourth. It is not likely, therefore, that their evolution can be explained by any one theory, even by one so broad in its scope as that of sexual selection.

If, for example, in the mammals a more vigorous male, due to greater development of the testes, were '^selected" by a female, the chances are that his secondary sexual characters will be better developed than are those of less vigorous males, but he is selected, not on this account, but because of his vigor. If a male bird were ^^ selected" on account of greater vigor, it does not appear that his secondary sexual characters would be more excessively developed than those of less vigorous males, provided that his vigor were due to the early or greater development of the testes. If in birds the male by selecting the female has brought about the suppression of the male plumage, which is their common inheritance, he must have done so by selecting those females whose ovaries produced the greatest amount of internal s'ecretions which suppresses male-feathering. Moreover, he must have selected, not fluctuating variations, but germinal variations. In insects the development of the secondary sexual characters is not connected with the condition of the reproductive organs, but is determined by the complex of factors that determines sex itself. If selection acts here, it must act directly on germinal variations, that are independent in origin of the sex-determining factor, but dependent on it for their expansion or suppression.

These considerations make many of the earlier statements appear crude and unconvincing ; for, they show that the origin of the secondary sexual characters is a much more complex affair than was formerly imagined.

These same considerations do not show, however, that if a new germinal character appeared that gave its possessor some advantage either by accelerating the opposite sex to quicker mating or by being correlated with greater vigor and thereby making more certain the discovery of a mate, such a character would not have a better chance of perpetuation. But in such a case, the emphasis no longer lies on the idea of selection with its emotional impHcations, but rather on the appearance of a more effective machine that has arisen, not because of selection, but, having arisen quite apart from any selective process, has found itself more efficient. Selection has always implied the idea that it creates something. Now that the evidence indicates that selection is not a guaranteed method of creating anything, its efficiency as a means of easy explanation is seriously impaired.


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Pages where the terms "Historic" (textbooks, papers, people, recommendations) 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, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Heredity and Sex (1913): 1 Evolution of Sex | 2 Mechanism of Sex-Determination | 3 Mendelian Principles of Heredity and Bearing on Sex | 4 Secondary Sexual Characters Relation to Darwin's Theory of Sexual Selection | 5 Effects of Castration, Transplantation on Secondary Sexual Characters | 6 Gynandromorphism, Hermaphroditism, Parthenogenesis, and Sex | 7 Fertility | 8 Special Cases of Sex-Inheritance | Bibliography


Morgan TH. Heredity and Sex (1913) Columbia University Press, New York.


Cite this page: Hill, M.A. (2020, August 6) Embryology Heredity and Sex (1913) 5. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Heredity_and_Sex_(1913)_5

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© Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G