Heredity and Sex (1913) 1

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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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Chapter I The Evolution of Sex

Animals and plants living to-day reproduce themselves in a great variety of ways. With a modicum of ingenuity we can arrange the different ways in series beginning with the simplest and ending with the more complex. In a word, we can construct systems of evolution, and we like to think that these systems reveal to us something about the evolutionary process that has taken place.

There can be no doubt that our minds are greatly impressed by the construction of a graded series of stages connecting the simpler with the complex. It is true that such a series shows us how the simple forms might conceivably pass by almost insensible (or at least by overlapping) stages to the most comphcated forms. This evidence reassures us that a process of evolution could have taken place in the imagined order. But our satisfaction is superficial if we imagine that such a survey gives much insight either into the causal processes that have produced the successive stages, or into the interpretation of these stages after they have been produced.

Such a series in the present case would culminate in a process of sexual reproduction with males and females as the actors in the drama. But if we are asked what advantage, if any, has resulted from the process of sexual reproduction, carried out on the two-sex scheme, we must confess to some uncertainty.

The most important fact that we know about living matter is its inordinate power of increasing itself. If all the fifteen million eggs laid by the conger eel were to grow up, and in turn reproduce, in two years the sea would be a wriggling mass of fish.

A single infusorian, produced in seven days 935 descendants. One species, stylonichia, produced in Gj/^ days a mass of protoplasm weighing one kilogram. At the end of 30 days, at the same rate, the number of kilograms would be 1 followed by 44 zeros, or a mass of protoplasm a million times larger than the volume of the sun.

Another minute organism, hydatina, produces about 30 eggs. At the end of a year (65 generations), if all the offspring survived, they would form a sphere whose limits would extend beyond the confines of the known universe.

The omnipresent English sparrow would produce in 20 years, if none died except from old age, so many descendants that there would be one sparrow for every square inch of the State of Illinois. Even slowbreeding man has doubled his numbers in 25 years. At the same rate there would in 1000 years not be standing room on the surface of the earth for his offspring.

I have not gone into these calculations and will not vouch for them all, but whether they are entirely correct or only partially so, they give a rough idea at least of the stupendous power of growth.

There are three checks to this process : First, the food supply is insufficient — you starve ; second, animals eat each other — you feed ; third, substances are produced by the activity of the body itself that interfere with its powers of growth — you poison yourself. The laws of food supply and the appetites of enemies are as inexorable as fate. Life may be defined as a constant attempt to find the one and avoid the other. But we are concerned here with the third point, the methods that have been devised of escape from the limitations of the body itself. This is found in reproduction. The simplest possible device is to divide. This makes dispersal possible with an increased chance of finding food, and of escaping annihilation, and at the same time by reducing the mass permits of a more ready escape of the by-products of the living machine.

Reproduction by simple division is a well-known process in many of the lower animals and plants ; it is almost universal in one-celled forms, and not unknown even in many-celled organisms. Amoeba and paramoecium are the stock cases for unicellular animals; many plants reproduce by buds, tubers, stolons, or shoots ; hydroids and sea-anemones both divide and bud ; many planarians, and some worms, divide transversely to produce two new individuals. But these methods of reproduction are limited to simple structures where concentration and division of labor amongst the organs has not been carried to an extreme. In consequence, what each part lacks after the division can be quickly made good, for delay, if prolonged, would increase the chances of death.

But there is another method of division that is almost universal and is utihzed by high and by low forms alike : individual cells, as eggs, are set free from the rest of the body. Since they represent so small a part of the body, an immense number of them may be produced on the chance that a few will escape the dangers of the long road leading to maturity. Sometimes the eggs are protected by jelly, or by shells, or by being transparent, or by being hidden in the ground or under stones, or even in the body of the parent. Under these circumstances the animal ventures to produce eggs with a large amount of food stored up for the young embryo.

So far reaching were the benefits of reproduction by eggs that it has been followed by almost every species in the animal and plant kingdom. It is adhered to even in those cases where the animals follow other grosser methods of separation at the same time. We find, however, a strange limitation has been put upon the process of reproduction by eggs. Before the egg begins its development it must be fertilized. Cells from two individuals must come together to produce a new one.

The meaning of this process has baffled biologists ever since the changes that take place during fertilization were first discovered ; in fact, long before the actual processes that take place were in the least understood. There is a rather extensive and antiquated literature dealing with the part of the male and of the female in the process of procreation. It would take us too far to attempt to deal with these questions in their historical aspects, but some of their most modern aspects may well arrest our attention.

In the simplest cases, as shown by some of the onecelled organisms, two individuals fuse into a single one (Fig. 1) ; in other related organisms the two individuals that fuse may be unequal in size. Sometimes we speak of these as male and female, but it is questionable whether we should apply to these unicellular types the same names that we use for the many-celled forms where the word sex applies to the soma or body, and not to the germ cells.

Fig. 1. — Union of two individuals (Stephanosphoera pluvialis) to form a single individual. (After D5flein.)

One of the best known cases of conjugation is that of paramcecium. Under certain conditions two individuals unite and partially fuse together. An interchange of certain bodies, the micronuclei, then takes place, as shown in Fig. 2, and in diagram. Fig. 3. The two conjugating paramcecia next separate, and each begins a new cycle of divisions. Here each individual may be said to have fertilized the other. The process recalls what takes place in hermaphroditic animals of higher groups in the sense that sperm from one individual fertilizes eggs of the other.

We owe to Maupas the inauguration of an epochmaking series of studies based on phenomena like this in paramoecium.

Fig. 2. — Conjugation in Parama?cium. The micronucleus in one individual is represented in black, in the other by cross-lines. The macronucleus in both is stippled. A-C, division of micronucleus into 2 and 4 nuclei; C^-D, elongation of conjugation nuclei, which interchange and recombine in E; F-J, consecutive stage in one ex-conjugant to show three divisions of new micronucleus to produce eight micronuclei (J). In lower part of diagram the first two divisions of the ex-conjugant {J) with eight micronuclei are shown, by means of which a redistribution of the eight micronuclei takes place. See also Fig. 100.

Fig. 3. — The nuclei of two individuals of paramoecium in I (homozygous in certain factors, and heterozygous in other factors), are represented as dividing twice (in II and III); the first division, II, is represented as reducing, i.e. segregation occurs ; the second division, III, is represented as efjuational, i.e. no reduction but division of factors, as in the next or conjugation division, IV, also.

Maupas found by following from generation to generation the division of some of these protozoa that the division rate slowly declines and finally comes to an end. He found that if a debilitated individual conjugates with a wild individual, the death of the race is prevented, but Maupas did not claim that through conjugation the division rate was restored. On the contrary he found it is lower for a time.

He also discovered that conjugation between two related individuals of these weakened strains produced no beneficial results.

Biitschli had earher (1876) suggested that conjugation means rejuvenation or renewal of youth, and Maupas' results have sometimes been cited as supporting this view. Later work has thrown many doubts on this interpretation and has raised a number of new issues.

In the first place, the question arose whether the decline that Maupas observed in the rate of division may not have been due to the uniform coi;iditions under which his cultures were maintained, or to an insufficiency in some ingredient of these cultures rather than to lack of conjugation. Probably this is true, for Calkins has show^n that by putting a declining race into a different medium the original division rate may be restored. Woodruff has used as culture media a great variety of food stuffs and has succeeded in keeping his lines without loss of vigor through 3000 generations. Maupas records a decline in other related protozoa at the end of a few hundred generations.

Butschli 's idea that by the temporary union (with interchange of micronuclei) of two weak individuals two vigorous individuals could be produced seems mysterious ; unless it can be made more explicit, it does not seem in accord with our physico-chemical conceptions. Jennings, who has more recently studied in greater detail the process of division and conjugation in paramoecium, has found evidence on which to base a more explicit statement as to the meaning of rejuvenescence through conjugation.

Jennings' work is safeguarded at every turn by careful controls, and owing in large part to these controls his results make the interpretations more certain. He found in a vigorous race, that conjugated at rather definite intervals, that after conjugation the division rate was not greater than it had been before, but on the contrary was slower — a fact known, as he points out, to Maupas and to Hertwig. Conjugation does not rejuvenate in this sense.

Jennings states that, since his race was at the beginning vigorous, the objection might be raised that the conditions were not entirely fulfilled, for his predecessors had concluded that it is a weakened race that was saved from annihilation by the process. In order to meet this objection he took some individuals from his stock and reared them in a small amount of culture fluid on a slide. After a time they became weakened and their rate of division was retarded. He then allowed them to conjugate, and reared the conjugants. Most of these were not benefited in the least by the process, and soon died. A few improved and began to multiply, but even then not so fast as paramcecia in the control cultures that had been prevented from conjugating. Still others gave intermediate rates of division.

He concludes that conjugation is not in itself beneficial to all conjugants, but that the essence of the process is that a recombination of the hereditary traits occurs as shown in the diagram, Fig. 3 and 4. Some of these new combinations are beneficial for special conditions — others not. The offspring of those conjugants that have made favorable combinations will soon crowd out the descendants of other conjugants that have made mediocre or injurious combinations. Hence, in a mass culture containing at all times large numbers of individuals, the maximum division rate is kept up, because, at any one time, the majority of the individuals come from the combinations favorable to that special environment.

Fig. 4. — Illustrating conjugation between two stocks, with pairs of factors A, B, C, D, and a, b, c, d ; and union of pairs into Aa, Bb, Cc, Dd. After these separate, their possible recombinations are shown in the 16 smaller circles. (After Wilson.)

There are certain points in this argument that call for further consideration. In a mass culture the favorable combinations for that culture will soon be made, if conjugation is taking place. At least this is true if such combinations are homogeneous (homozygous, in technical language). Under such circumstances the race will become a pure strain, and further conjugation could do nothing for it even if it were transferred to a medium unsuited to it.

In the ordinary division of a cell every single determiner divides and each of the new cells receives half of each determiner. Hence in the case of paramoecium all the descendants of a given paramoecium that are produced by division must be exactly alike. But in preparation for conjugation a different process may be supposed to take place, as in higher animals, among the determiners. The determiners unite in pairs and then, by division, separate from each other. Fig. 4. In consequence the number of determiners is reduced to half. Each group of determiners will be different from the parent group, provided the two determiners that united were not identical. If after this has occurred conjugation takes place, the process not only restores the total number of determiners in each conjugant, but gives new groups that differ from both of the original groups.

The maintenance of the equilibrium between an organism and its environment must be a very delicate matter. One combination may be best suited to one environment, and another combination to another. Conjugation brings about in a population a vast number of combinations, some of which may be suited to the time and place where they occur. These survive and produce the next generation.

Jennings' experiments show, if I understand him correctly, that the race he used was not homogeneous in its hereditary elements ; for when two individuals conjugated, new combinations of the elements were formed. It seems probable, therefore, that the chemical equilibrium of paramoecium is maintained by the presence of not too much of some, or too little of other, hereditary materials. In a word, its favorable combinations are mixed or heterozygous.

The meaning of conjugation, and by implication, the meaning of fertilization in higher forms is from this point of view as follows : — In many forms the race, as a whole, is best maintained by adapting itself to a widely varied environment. A heterozygous or hybrid constitution makes this possible, and is more likely to perpetuate itself in the long run than a homozygous race that is from the nature of the case suited to a more limited range of external conditions.

What bearing has this conclusion on the problem of the evolution of sex and of sexual reproduction ?

This is a question that is certain to be asked. I am not sure that it is wise to try to answer it at present, in the first place because of the uncertainty about the conclusions themselves, and in the next place, because, personally, I think it very unfair and often very unfortunate to measure the importance of every result by its relation to the theory of evolution. But with this understanding I may venture upon a few suggestions.

If a variation should arise in a hermaphroditic species (already reproducing sexually) that made crossfertilization more likely than self-fertilization, and if, as a rule, the hybrid condition (however this may be explained) is more vigorous in the sense that it leaves more offspring, such a variation would survive, other things being equal.

But the establishment of the contrivance in the species by means of which it is more likely to crossfertilize, might in another sense act as a drawback. Should weak individuals appear, they, too, may be perpetuated, for on crossing, their weakness is concealed and their offspring are vigorous owing to their hybrid condition. The race will be the loser in so far as recessive or weak combinations will continue to appear, as they do in many small communities that have some deficiency in their race ; but it is a question whether the vigor that comes from mixing may not more than compensate for the loss due to the continual appearance of weakened individuals.

This argument applies to a supposed advantage within the species. But recombination of what already exists will not lead to the development of anything that is essentially new. Evolution, however, is concerned with the appearance and maintenance of new characters. Admitting that sexual reproduction proved an advantage to species, and especially so when combined with a better chance of cross-fertilization, the machinery would be at hand by means of which any new character that appeared would be grafted, so to speak, on to the body of the species in which it appeared. Once introduced it would be brought into combination with all the possible combinations, or races, already existing within the species. Some of the hybrid combinations thus formed might be very vigorous and would survive. This reasoning, while hypothetical, and, perhaps not convincing, points at least to a way in which new varieties may become incorporated into the body of a species and assist in the process of evolution.

It might be argued against this view that the same end would be gained, if a new advantageous variation arose in a species that propagated by non-sexual methods or in a species that propagated by self-fertilization. The offspring of such individuals would transmit their new character more directly to the offspring. Evolution may, of course, at times have come about in this way, and it is know^n that in many plants selffertilization is largely or exclusively followed. But in a species in which cross-fertilization was the established means of propagation, the new character would be brought into relation with all the other variations that are found in the component races and increase thereby its chances of favorable combinations. We have in recent years come to see that a new heritable character is not lost by crossing, or even weakened by blending," as was formerly supposed to be the case; hence no loss to the character itself will result in the union with other strains, or races, within the species.

If then we cannot explain the origin of sexual reproduction by means of the theory of evolution, we can at least see how the process once begun might be utiHzed in the building up of new combinations ; and to-day evolution has come to mean not so much a study of the origination of new characters as the method by which new characters become estabhshed after they have appeared.

The Body and the Germ-Plasm

As I have said, it is not unusual to speak of the unicellular animals and plants as sexual individuals, and where one of them is larger than the other it is sometimes called the female and the smaller the male. But in many-celled animals we mean by sex something different, for the term apphes to the body or soma, and not to the reproductive cells at all. The reproductive cells are eggs and sperm. It leads to a good deal of confusion to speak of the reproductive cells as male and female. In the next chapter it will be pointed out that the eggs and sperm carry certain materials ; and that certain combinations of these materials, after fertilization has occurred, produce females ; other combinations produce males ; but males and females, as such, do not exist until after fertilization has taken place.

The first step, then, in the evolution of sex was taken when colonies of many cells appeared. We find a division of labor in these many-celled organisms ; the germ-cells are hidden away inside and are kept apart from the wear and tear of life. Their maintenance and protection are taken over by the other cells of the colony. Even among the simplest colonial forms we find that some colonies become specialized for the production of small, active germ-cells. These colonies are called the males, or sperm-producing colonies. The other colonies specialize to produce larger germ-cells — the eggs. These colonies are called females or egg-producing colonies. Sex has appeared in the living world.

To-day we are only beginning to appreciate the farreaching significance of this separation into the immortal germ-cells and the mortal body, for there emerges the possibility of endless relations between the body on the one hand and the germ-cells on the other. Whatever the body shows in the way of new characters or new ways of reacting must somehow be represented in the germ-cells if such characters are to be perpetuated. The germ-cells show no visible modification to represent their potential characters. Hence the classical conundrum — whether the hen appeared before the egg, or the egg before the hen ? Modern biology has answered the question with some assurance. The egg came first, the hen afterwards, we answer dogmatically, because we can understand how any change in the egg will show itself in the next generation — in the new hen, for instance ; but despite a vast amount of arguing no one has shown how a new hen could get her newness into the old-fashioned eggs.

Few biological questions have been more combated than this attempt to isolate the germ-tract from the influence of the body. Nussbaum was amongst the first, if not the first, to draw attention to this distinction, but the credit of pointing out its importance is generally given to Weismann, whose fascinating speculations start from this idea. For Weismann, the germcells are immortal — the soma alone has the stigma of death upon it. Each generation hands to the next one the immortal stream unmodified by the experience of the body. What we call the individual, male or female, is the protecting husk. In a sense the body is transient — temporary. Its chief ^^ purpose" is not its individual life, so much as its power to support and carry to the next point the all important reproductive material.

Modern research has gone far towards establishing Weismann's claims in this regard. It is true that the germ-plasm must sometimes change — otherwise there could be no evolution. But the evidence that the germplasm responds directly to the experiences of the body has no substantial evidence in its support. I know, of course, that the whole Lamarckian school rests its argument on the assumption that the germ-plasm responds to all profound changes in the soma ; but despite the very large literature that has grown up dealing with this matter, proof is still lacking. And there is abundant evidence to the contrary.

On the other hand, there is evidence to show that the germ-plasm does sometimes change or is changed. Weismann's attempt to refer all such changes to recombinations of internal factors in the germ-plasm itself has not met with much success. Admitting that new combinations may be brought about in this way, as explained for paramoecium, yet it seems unlikely that the entire process of evolution could have resulted by recombining what already existed ; for it would mean, if taken at its face value, that by recombination of the differences already present in the first living material, all of the higher animals and plants were foreordained. In some way, therefore, the germplasm must have changed. We have then the alternatives. Is there some internal, initial or driving impulse that has led to the process of evolution ? Or has the environment brought about changes in the germplasm ? We can only reply that the assumption of an

Fig. 5. — Schematic representation of the processes occurring during the fertilization and subsequent segmentation of the ovum. (Boveri, from Howell.)

internal force puts the problem beyond the field of scientific explanation. On the other hand, there is a small amount of evidence, very incomplete and insufficient at present, to show that changes in the environment reach through the soma and modify the germinal material.

It would take us too far from our immediate subject to attempt to discuss this matter, but it has been necessary to refer to it in passing, for it lies at the foundation of all questions of heredity and even involves, as we shall see later, the question of heredity of sex.

This brings us back once more to the provisional conclusion we reached in connection with the experiments on paramoecium. When the egg is fertihzed by the sperm, Fig. 5, the result is essentially the same as that which takes place when two paramcecia fertiUze each other. The sperm brings into the egg a nucleus that combines with the egg-nucleus. The new individual is formed by recombining the hereditary traits of its two parents.

It is evident that fertilization accomplishes the same result as conjugation. If our conclusion for paramoecium holds we can understand how animals and plants with eggs and sperm may better readjust themselves now to this, now to that environment, within certain limits. But we cannot conclude, as I have said, that this process can make any permanent contribution to evolution. It is true that Weismann has advanced the hypothesis that such recombinations furnish the materials for evolution, but as I have said there is no evidence that supports or even makes plausible his contention.

I bring up again this point to emphasize that while the conclusion we arrived at — a provisional conclusion at best — may help us to understand how sexual reproduction might be beneficial to a species in maintaining itself, it cannot be utilized to explain the progressive advances that we must beUeve to have taken place during evolution.

The Early Isolation of the Germ-Cells

There is much evidence to show that the germ-cells appear very early in the development of the individual when they are set aside from the cells that differentiate into the body cells. This need not mean that the germcells have remained unmodified, although this is at first sight the most natural interpretation. It might be said, indeed, that they are among the first cells to differentiate, but only in the sense that they specialize, as germ-cells.

In a parasitic worm, ascaris, one of the first four cells divides differently from the other three cells. As seen in Fig. 6, this cell retains at its division all of its chromatin material, while in the other three cells some of the chromatin is thrown out into the cell-plasm. The single cell that retains all of the chromatin in its nucleus gives rise to the germ-cells.

Fig. 6. — Chromatin diminution and origin of the germ-cells in Ascaris. (After Boveri.)

In a marine worm-like form, sagitta, two cells can easily be distinguished from the other cells in the wall of the digestive tract (Fig. 7). They leave their first position and move into the interior of the body, where they produce the ovary and testes.

Fig. 7. — Origin of germ-cells in Sagitta. (From Korschelt and Heider.)

In several of the insects it has been shown that at a very early stage in the segmentation, one, or a few cells at most, lying at one end of the egg develop almost independently of the rest of the embryo (Fig. 8). Later they are drawn into the interior, and take up their final location, where they give rise to the germ-cells.

Fig. 8. — Origin of germ-cells in Miastor. Note small black protoplasmic area at bottom of egg into which one of the migrating segmentation nuclei moves to produce the germ-cells. (After Kahle.)

Even in the vertebrates, where, according to the earlier accounts, the germ-cells were described as appearing late in embryonic development, it has been shown that the germ-cells can be detected at a very early stage in the walls of the digestive tract (Fig. 9). Thence they migrate to their definitive position, and give rise to the cells from which the eggs or the sperm arise.

Fig. 9. — Origin of germ-cells in certain vertebrates, viz. turtle, frog, gar-pike and bow-fin. The germ-cells as darker cells are seen migrating from the digestive tract (endoderm). (After Allen.)

The germ-cells are in fact often the earliest cells to speciaUze in the sense that they are set aside from the other cells that produce the soma or body of the individual.

The Appearance of the Accessory Organs of Reproduction

As animals became larger the problem of setting free the germ-cells was a matter of great importance. Systems of outlets arose — the organism became piped, as it were. In the lower animals the germ-cells are brought to the surface and set free directly, and fertiUzation is a question of the chance meeting of sperm and egg ; for there is practically no evidence to show that the sperm is attracted to the egg and much evidence that it is not. Later, the copulatory organs were evolved in all the higher groups of animals by means of which the sperm of the male is transferred directly to the female. This makes more certain the fertihzation of the egg.

In the mollusks, in the insects and crustaceans, and in the vertebrates the organs of copulation serve to hold the individuals together during the act of mating, and at the same time serve to transfer the semen of the male to the oviduct, or to special receptacles of the female. Highly elaborated systems of organs and special instincts, no less elaborate, serve to make the union possible. In some types mating must occur for each output of eggs, but in other cases the sperm is stored up in special receptacles connected with the ducts of the female. From these receptacles a few sperm at a time may be set free to fertilize each egg as it passes the opening of the receptaculum. In the queen bee enough sperm is stored up to last the queen for five or six years and enough to fertilize a million eggs.

Fig. 10. — Squid : Two upper right-hand figures illustrate two methods of copulation. Lower right-hand figure dissected to show spermatophore placed in mantle cavity of female. Left-hand figure (below), spermatophore pocket behind mouth of male; upper figure, section of same. (After Drew.)

There are a few cases where the transfer from the male to the female is brought about in a different way. The most striking cases are those of the squids and octopi, and of the spiders.

In the squid, the male and fe.male interlock arms (Fig. 10). The male takes the packets of sperm (that are emitted at this time from the sperm-duct) by means of a special arm, and transfers the packets either to a special receptacle within the circle of arms of the female, or plants them within the mantle chamber itself of the female. Each packet of spermatozoa is contained in a long tube. On coming in contact with sea water the tube everts at one end, and allows the sperm to escape.

Fig. 11. — Octopus, male showing hectocotyl arm (ha). Copulation (below), small male, :>4.; large female, B.

After separation the female deposits her strings of eggs, which are fertilized by the sperm escaping from the spermatophores. In octopus and its allies, one arm, that is used to transfer the spermatophores, is specially modified at the breeding season (Fig. 11).

This arm is inserted by the male, as shown in the figure, within the mantle chamber of the female. In some species, Argonaut a argo for instance (Fig. 12), the arm is broken ofT, and remains attached by its suckers inside the mantle of the female. The eggs are later fertilized by sperm set free from this hectocotylized " arm.

Fig. 12. — Argonauta showing developing (A) and developed (B) hectocotyl arm, which, after being charged with spermatophores, is left in mantle of female.

The Secondary Sexual Characters

In the most highly evolved stages in the evolution of sex a new kind of character makes its appearance. This is the secondary sexual character. In most cases such characters are more elaborate in the male, but occasionally in the female. They are the most astonishing thing that nature has done : brilliant colors, plumes, combs, wattles, and spurs, scent glands (pleasant and unpleasant) ; red spots, yellow spots, green spots, topknots and tails, horns, lanterns for the dark, songs, bowlings, dances and tourneys — a medley of odds and ends.

The most familiar examples of these characters are found in vertebrates and insects, while in lower forms they are rare or absent altogether. In mammals the horns of the male stag are excellent examples of secondary sexual characters. The male sea cow is much greater in size than the female, and possesses long tusks. The mane of the lion is absent in the lioness.

Fig. 13. — Great bird of Paradise, male and female. (After Elliot.)

In birds there are many cases in which the sexes differ in color (Figs. 13 and 14). The male is often more brilliantly colored than the female and in other cases the nuptial plumage of the male is quite different from the plumage of the female. For example, the black and yellow colors of the male bobolink are in striking contrast with the brown-streaked female (Fig. 15). The male scarlet tanager has a fiery red plumage with black wings, while the female is olive green. The male of the mallard duck has a green head and a reddish breast (Fig. 16), while the female is streaked with brown. In insects the males of some species of beetles have horns on the head that are lacking in the female (Fig. 17). The males of many species of butterflies are colored differently from the females.

Fig. 14. — White-booted humming bird, two males and one female. (After Gould.)

The phosphorescent organ of our common firefly, Photinus pyralis, is a beautiful illustration of a secondary sexual character. On the under surface of the male there are two bands and of the female there is a single band that can be illuminated (Fig. 18). At night the males leave their concealment and fly about. A little later the females ascend to the tops of blades of grass and remain there without glowing. A male passes by and flashes his hght ; the female flashes back. Instantly he turns in his course to the spot whence the signal came and alights. He signals again. She replies. He ascends the blade, and if he cannot find her, he signals again and she responds. The signals continue until the female is found, and the drama of sex is finished.

Fig. 15. — Male and female bobolink. (From "Bird Lore.")

Fig. 16. — Male and female mallard duck. (From "Bird Lore.")

FiG. 17. — Male and female Hercules beetle. (After Kingsley.)

Mast has recently shown that the female firefly does more than simply respond to the signal of the male. If a male flies above and to the right of the female, she bends her abdomen so that its ventral surface is turned upward and to the right. If the male is above and to the left, the light is turned in this direction. If the male is directly above, the abdomen of the female is twisted almost upward. But if the male is below her, she emits her light without turning the body. In the firefly the evidence that the phosphorescent organ is of use in bringing the sexes together seems well established.

Fig. 18. — Male and female firefly.

Whether all secondary sexual organs are useful in mating is a question that must be referred to a later chapter.

The Sexual Instincts

Side by side with the evolution of these many kinds of structural difference the sexual instincts have evolved. It is only in the lowest forms that the meeting of the egg and sperm is left to chance. The instincts that bring the males and females together at the mating season, the behavior of the individuals at this time in relation to each other, forms one of the most curious chapters in the evolution of sex, for it involves courtship between the males and females ; the pairing or union of the sexes and subsequently the building of the nest, the care, the protection and feeding of the young, by one or both parents. The origin of these types of behavior is part of the process of evolution of sex ; the manner of their transmission in heredity and their segregation according to sex is one of the most difficult questions in heredity — one about which nothing was known until within recent years, when a beginning at least has been made.

A few samples taken almost at random will illustrate some of the familiar features in the psychology of sex. Birds have evolved some of the most complicated types of courtship that are known. It is in this group, too, as we have seen, that the development of secondary sexual characters has reached perhaps its highest types. But it is not necessarily in the species that have the most striking differences between the sexes that the courtship is most elaborate. In pigeons and their allies, for example, the courtship is prolonged and elaborate, yet the males and females are externally almost indistinguishable ; while in the barnyard fowl and in ducks the process is relatively simple, yet chanticleer is notoriously overdressed.

Even in forms so simply organized as the fishes it is known that the sexual instincts are well developed. In the common minnow, fundulus, the males develop in the breeding season elaborate systems of tactile organs. The male swims by the side of the female, pressing his body against her side, which causes her to set free a few eggs. At the same time the male sets free the sperm, thereby increasing the chance that some of the spermatozoa will reach the egg.

In bees, the sexual life of the hive is highly specialized. Mating never occurs in the hive, but when the young queen takes her nuptial flight she is followed by the drones that up to this have led an indolent and useless life in the colony. Mating occurs high in the air. The queen goes to the new nest and is followed by a swarm of workers who construct for her a new home. Here she remains for the rest of her life, fed and cared for by the workers, who give her the most assiduous attention — an attention that might be compared to courting were it not that the workers are not males but only immature females. The occurrence of these instincts in the workers that never leave or rarely at least leave offspring of their own is a special field of heredity about which we can do little more than speculate. This much, however, may be hazarded. The inheritance of the queen and of the worker is the same. We know from experimental evidence that the amount of food given to the young grub, when it hatches from the egg, is the external agent that makes the grub a queen or a worker. In the worker the sex glands are little developed. Possibly their failure to develop may in part account for the different behavior of the workers and of the queen. I shall devote a special chapter to this question of the influence of the secretions of the sex glands or reproductive organs on the character of the body. We shall see that in some animals at least an important relation exists between them.

In the spiders the mating presents a strange spectacle.

Let us follow Montgomery's careful observations on Phidippus purpuratus. The male spun a small web of threads from the floor to one side of his cage at an angle of 45°. 'Tour minutes later he deposited a minute drop of sperm on it, barely visible to the naked eye ; then extending his body over the web reached his palpi downwards and backwards, applying them alternately against the drop ; the palpal organs were pressed, not against the free surface of the drop, but against the other side of the web." Later, a minute drop of sperm is found sticking to the apex of one of the palpi. In 1678 Lister had shown that the male applies his palpi to the genital aperture of the female ; but not until 1843 was it found by Menge that the palpi carry the sperm drop.

In man, courtship may be an involved affair. Much of our literature revolves about this period, while painting and sculpture take physical beauty as their theme. Unsatiated with the natural differences that distinguish the sexes, man adds personal adornment which reaches its climax in the period of courtship, and leaves a lasting impression on the costuming of the sexes. Nowhere in the animal kingdom do we find such a mighty display ; and clothes as ornaments excel the most elaborate developments of secondary sexual characters of creatures lower in the scale.

I have sketched in briefest outline some of the general and more familiar aspects of sex and the evolution of the sexes. In the chapters that follow we shall take up in greater detail many of the problems that have been only touched upon here.

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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. (2021, April 23) Embryology Heredity and Sex (1913) 1. Retrieved from

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