Hormones and Heredity / A Discussion of the Evolution of Adaptations and the Evolution of Species

by J. T. Cunningham

The case of Dasyurus, however, seems inconsistent with this argument, for, as previously mentioned, Sandes found that in this Marsupial the corpora lutea persisted during the greater part of the period of lactation, which continues for four months after parturition. During the whole of this time there are no embryos in the uteri, and therefore it might be urged absorption of hormones from the embryos cannot be the cause of the persistence of corpora lutea in pregnancy. But it seems to me that a complete answer to this objection is supplied by the peculiar relations of the embryos to the pouch in Dasyurus and other Marsupials. The skin of the pouch while the embryos are in it is very soft, congested, and glandular; at the same time the embryos when transferred to the pouch at parturition are very small, immature, and have a soft delicate skin. The relation of embryos to pouch in Dasyurus, therefore, is closely similar to that of embryos to uterus after the first few days of pregnancy in the Eutheria. It is true there is no placenta, but the mouths of the embryos are in very close contact with the teats, and both the skin of the embryos and that of the pouch are soft and moist. If any special substances are given off by the embryos in the uterus in ordinary gestation, the same substances would continue to be given off by the embryos in the marsupial pouch, and these must be absorbed by the skin of the pouch. In this way it seems to me we have a logical explanation of the fact that the corpora lutea in the Marsupial are not absorbed at parturition as in Eutheria. As Sandes says the 'greater part of the period of lactation,' it would appear that absorption of the corpora lutea takes place when the young Dasyurus have grown to some size, become covered with hair, and are able to leave the teats or even the pouch at will. Under these conditions it is obvious that diffusion of chemical substances from the young through the walls of the pouch would come to an end. It would be interesting in this connexion to know more of the relation of egg and embryo to the pouch and to the corpora lutea in Echidna. In Ornithorhynchus the eggs are hatched in a nest and there is no pouch.

On this view that the corpora lutea are the result, not the cause, of intra-uterine gestation, it would no longer be possible to maintain the theory that the corpus luteum in the human species is the cause by its internal secretion of the phenomenon of menstruation. This was the theory of Born and Fränkel. [Footnote: See Biedl, Internal Secretory Organs (Eng. trans.), 1912, p. 404.] Biedl's conclusion is that the periodic development and disintegration of the uterine mucous membrane in the menstrual cycle is due to the hormone of the interstitial cells of the ovary. Leopold and Ravana found that ovulation as a rule coincides with menstruation, but may take place at any time. Here, again, the problem must be considered from the point of view of evolution. It can scarcely be doubted that the thickening and growth of the mucous membrane in the menstrual cycle is of the same nature as that which takes place in pregnancy. When the ovum or ova are not fertilised the development comes to an end after a certain time, differing in different species of Mammals, and the membrane sloughs, returns to its original, state, and then begins the same process of development again.

Menstruation, then, must be interpreted as an abortive parturition, both in woman and lower Mammals, though in the latter it is not usually accompanied by hemorrhage, and is called pro-oestrus. The question then to be considered is, what determines parturition and menstruation? The presence of the fertilised ovum must have been the original cause of the hypertrophy of the uterine mucous membrane, and in its congenital or hereditary development the chemical substances diffusing from the ova in the uterus or even in the Fallopian tube may well be the stimulus starting the hypertrophy. But what determines the end of the pregnancy? Is it merely the increasing distension of the uterus by the developing foetus? This could scarcely be the case in the Marsupials in which the foetus when born is quite minute. Nor can we attribute parturition to renewed ovulation, for this occurs in Dasyurus only once a year. All we can suggest at present is that a certain periodic development takes place by heredity in presence of the hormones exuded by the fertilised ovum and the embryo developed from it. When the ovum or ova, not being fertilised, die the period of development is (usually) shortened and pro-oestrus or menstruation occurs. In the dog, however, the period of the oestrus cycle is about the as that of gestation—namely, six months.

The so-called descent of the testicles occurs exclusively in Mammals, in which with a few important exceptions it is universal. This is a very remarkable case of the change of position of an organ in the course of development. The original position of the testis on either side is quite similar to that of the same organ in birds or reptiles. The genital ridge runs along the inner edge of the mesonephros, with which the testicular tubules become connected. The testis, with the mesonephros, forming the epididymis, closely attached to it, projects into the coelom, and without losing its connexion with the peritoneum changes its position gradually during development, passing backwards and downwards until it comes to lie over the wall of the abdomen just in front of the pubic symphysis of the pelvic girdle. There the abdominal wall on either side of the middle line becomes thin and distended to form a pouch, the scrotal sac, into which the testis passes, still remaining attached to the peritoneum which lines the pouch, while the distal end of the vas deferens retains its original connexion with the urethra. The movement of the testis can thus be accurately described as a transposition or dislocation.

Various causes have been suggested for the formation of the scrotum, but no one has ever been able to suggest a use for it. It has always been quite impossible to bring it within the scope of the theory of natural selection. The evolution of it can only be explained either on the theory of mutation or some Lamarckian hypothesis. The process of dislocation of the testis does not conform to the conception of mutation, nor agree with other cases of that phenomenon. A mutation is a change of structure affecting more or less the whole soma, but showing itself especially in some particular organ or structure. But I know of no mutation occurring under observation which consisted, not in a change of structure or function, but merely in a change of position of an organ from one part of the body to another, and moreover a change which takes place by a continuous process in the course of development. If the testes were developed from the beginning in a different part of the abdomen, there might be some reason in calling the change a mutation. Moreover, if it is a mutation, why has it never occurred in any other class of Vertebrates except Mammals?

In 1903 Dr. W. Woodland published [Footnote: Proc. Zool. Soc., 1903, Part 1.] a Lamarckian theory of this mammalian feature, the probability of which it seems to me has been increased rather than decreased by the progress of research concerning heredity and evolution since that date. Dr. Woodland correlated the dislocation of the testes with the special mechanical features of the mode of locomotion in Mammalia. His words are: 'The theory here advocated is to the effect that the descent of the testes in the Mammalia has been produced by the action of mechanical strains causing rupture of the mesorchial attachments, such strains being due to the inertia of the organs reacting to the impulsiveness involved in the activity of the animals composing the group.' The 'impulsiveness' is the galloping or leaping movement which is characteristic of most Mammals when moving at their utmost speed, as seen, for example, in horses, deer, antelopes, dogs, wolves, and other Ungulata and Carnivora. It is obvious that when the body is descending to the ground after being hurled upwards and forwards, the abdominal organs have acquired a rapid movement downwards and forwards; when the body reaches the ground its movement is stopped suddenly, while the abdominal organs continue to move. The testes therefore are violently jerked downwards away from their attachments and at the same time forward. The check to the forward movement, however, is momentary, while the body is immediately thrown again upwards and forwards, which by the law of inertia means that the testes are thrown still more downwards and backwards. There is no reason to suppose, as Dr. Woodland suggests, that any rupture of the mesorchium was the usual result of these strains, but a constant pull or tension was caused in the direction in which the testes actually move during development. On this theory we have to consider (1) how such strains could cause a shifting of the peritoneal attachment, (2) why the testes should be supposed to be particularly affected more than other abdominal organs. The answer to the first question is that the strains would cause a growth of the connecting membrane (mesorchium) at the posterior end, accompanied by an absorption of it at the anterior end. The answer to the second question is that the testes are at once the most compact and heaviest organs in the abdomen, and at the same time the most loosely attached. The latter statement does not apply to the mesonephros or epididymis which has moved with the testis, but the latter cannot function without the former, and it may be supposed that the close attachment of the epididymis to the testis had come about in the early Mammalia before the change of position was evolved.

It is evident that the violent shocks of the galloping or leaping movement do not occur in Birds, Reptiles, or Amphibia. Ostriches run very fast and do not fly, but their progression is a stride with each foot alternately, not a gallop. The Anura among the Amphibia are saltatory, but their leaps are usually single, or repeated only a few times, not sustained gallops. The exceptions among the Mammalia still more tend to prove the close correspondence between the 'impulsive' mode of progression and the dislocation of the male gonads. In the Monotremata there is no scrotum, the testes are in a position similar to that which obtains in Reptiles, and they are the only Mammals in which these organs are anterior to the kidneys. In locomotion they are sluggish, there is no running or galloping among them. Ornithorhynchus is aquatic in its habits, and Echidna is nocturnal and moves very slowly. In Marsupials the scrotum is in front of the penis, but really in the same position as in other Mammals—that is, in front of the ventral part of the pelvic girdle. It is the penis which is different, as the skin around the organ has not united in a ventral suture below it, while the organ itself has not grown forward adnate to the abdominal skin as in most other Mammals. The scrotum is always anterior to the origin of the penis, although in the Eutheria apparently behind that organ. The larger Marsupials like the kangaroos are eminently saltatory, and the others are active in locomotion. The aquatic Mammals Sirenia and Cetacea have no scrotum, the testes being abdominal. It is unnecessary to inquire whether this is the original position, or whether they are descended from ancestors which had a scrotum: in either case the position of the testes corresponds to the absence of what Dr. Woodland calls impulsiveness in progression. The Fissipedia offer an instructive example, for while the Otariidae have the hind feet turned forward and can move on land somewhat like ordinary Mammals, the Phocidae cannot move their hind legs independently or turn them forward, and can only drag themselves about on land for short distances. In the former the testes are situated in a well-defined scrotum, in the latter these organs are abdominal. The Phocidae are probably descended from Mammals of the terrestrial type with a scrotum, which has disappeared in the course of evolution. Perhaps the most curious exception is that of the elephants, in which the testes are abdominal. Here, in consequence of their structure and massive shape, locomotion in usually a walk, and though they run occasionally the gait is a trot, not a sustained gallop, and leaping is out of the question. Sloths which hang from branches upside down have abdominal testes, but even here they are in a posterior position, between, the rectum and the bladder, so there has apparently been a degree of dislocation, probably inherited from ancestors with more terrestrial habits.

The fact that the ovaries do not occupy normally a position similar to that of the testes is in accordance with the theory, for they are very much smaller than the testes; and yet they have undergone some change of position, for they are posterior to the kidneys.

The facts agree with the hormone theory, for it is to be noted that although the development of the scrotum is confined to the males, the 'descent' or dislocation takes place in the foetus, and not at the period of puberty. This is in accordance with the fact that the mechanical conditions to which the change is attributed are not related to sexual habits, but to the general habits of life which begin soon after birth. The development, therefore, may be considered to be related to the presence of a hormone derived from the normal testis, but not to a special quantity or quality of hormone associated with maturity or the functional activity of the organ. In Rodents, however, there is a difference in the organs, not only at maturity, but in every rutting season, at any rate in Muridae such as rats and others. In the rutting season the testes become much larger and descend into the scrotal sacs, at other times of the year being apparently more or less abdominal. In rabbits and hares, which have a much more impulsive progression, the organs seem to be always in the scrotal sacs.

It might be thought that in this case, although the hormone theory of heredity might be applied, there was no reason to suppose that a hormone derived from the testis in the individual development was necessary in order that the hereditary change should take place. If the individual was male and therefore had a testis, this organ would by heredity go through the process of dislocation. But there is the curious fact that when the descent is not normal and complete, in what is called cryptorchidism, the organs are always sterile. The retention of the testes within the abdomen may be regarded as a case of arrested development, like many other abnormalities, but this does not explain why the retained testes should always be sterile, without spermatogenesis. If the inherited or congenital process of dislocation requires the presence of hormones produced by a normal testis, then we can understand why a defective testis does not descend completely, because it does not produce the hormone which is necessary to stimulate the hereditary mechanism to complete dislocation. It is often stated that in cryptorchidic individuals the sexual instincts and somatic sexual characters are well developed, which would appear contradictory to the above explanation, but according to Ancel and Bouin such individuals in the case of the pig show considerable differences in the secondary signs of sex and in the external genital organs, presenting variations which lie between the normal and the castrated animal.