Popular Science Monthly/Volume 61/May 1902/The Physical Basis of Heredity
|THE PHYSICAL BASIS OF HEREDITY.|
By Professor CARL H. EIGENMANN,
'HE is a chip of the old block' is the popular expression used in applying the best known general law of heredity that 'like begets like' to a particular case. But in saying so we state but half the truth. The chip is like the block and not like the block. Each individual is unique; no two were ever cast in the same mold.
There are always two phenomena associated in the development of a new individual. One is the repetition in the offspring of characters possessed by his ancestors, either near or remote. The second is the formation of new characters which have never appeared before in any individual. I shall confine myself to the first of these phenomena.
What characters has any individual inherited from his ancestors and what ones are new to him is always a question of the first consideration in a study of heredity. The consideration of which of these he may transmit to his offspring naturally follows.
He has always inherited and always transmits those characters that distinguish his species, race or family from other species, races or families. A backbone, four limbs and hairiness are always inherited and transmitted by a mammal, as backbone, four limbs and feathers are always transmitted by a bird. The erect position, peculiarities of hand and foot and those other features which together make a man are always inherited and always transmitted. The inheritance and transmission of the racial characters of the Jews distinguish them from all the various peoples with which they are found. The same is true of the Chinese, Indians, Negroes and, to a less degree, of the less pure races of Teutons and Anglo-Saxons.
Aside from characters that are always inherited and transmitted, there are groups of characters that may have been inherited and that are transmissible, but that are not necessarily transmitted. It is the peculiar combination of some of these characters that constitutes family traits. These transmissible, but not necessarily transmitted, characters may be anatomical and range from the height of the individual to such minute details as moles, a few long hairs in the eyebrows or even smaller details.
They may be physiological. Longevity is transmissible; so are a tendency to fatness and leanness and, in the males of certain families, a tendency to baldness. The members of some families have a tendency to become corpulent at a certain period, and no reasonable amount of starving seems to affect this tendency. After half the inmates of Andersonville prison had made their escape, one of these hereditarily stout individuals even after weeks of starving became fastened in the tunnel, preventing the escape of the rest of the prisoners. The members of other families remain 'hungry Cassiuses' however well they may be fed.
Mental peculiarities are transmissible. Sometimes one mental trait of the parents is transmitted to one child while others are transmitted to another. Weit Bach, a baker who lived in the middle of the sixteenth century, was a mild form of a musical prodigy. He transmitted his musical talent through at least eight generations, and hundreds of descendants, twenty-nine of whom became famous musicians. The case of the Jukes family is well known. In the course of six generations of descendants from one woman 52% of the female descendants became public women, 23% of the children were illegitimate. There were seven times more paupers among the women than among all women, and nine times as many among the men.
We have, then, characters that are always transmitted and characters that may be transmitted. As a third group we have characters concerning which we have doubt, and, at present, much discussion. This third group of characters includes individual peculiarities which have not been inherited, but are acquired during the lifetime of the individual as the result of his education, his activities and the effect of the climate and other elements of his environment. Whether or not these are transmissible has. been the question of the past ten years. The discussion was started by Weismann, who denied, for theoretic reasons, that any of the characters so produced are transmissible. It is agreed that the individual characters that result from accidental or voluntary mutilations are not transmitted. Wooden legs are never transmitted. Wooden heads sometimes are! Many instances have been brought forward to prove the transmissibility of acquired characters, but none of these cases has been accepted as conclusive. From my own studies of the effect of disuse on eyes and the absence of light on color, I am convinced that the results of the activities and the characters due to the environment are transmissible. Mehnert goes so far as to maintain that races are progressing in so far as marriage does not take place until comparatively late in life, and children are not born till the molding effects of activities and environment have individualized the parents. Sargent, of the Harvard gymnasium, evidently convinced of the transmissibility of acquired characters, advises the delay of marriage among Cubans until full maturity, as a means of raising their physical standard. He found that at present the average Cuban man is, in size and weight, the equal of the American female student.
An illustration of environment, including education, overcoming hereditary tendencies has recently been brought to my notice by Mr. A. J. Redmon. Mr. Redmon reared two murderous sparrow-hawks in a cage with young larks and wrens. "They all grew up together; the little wrens would creep under the sparrow-hawks for protection at night. The two hawks never attempted to hurt the larks or wrens." Mr. Redmon tried to starve the hawks into killing birds, but they utterly refused to disgrace their education.
Galton has determined just how much, on an average, each ancestor contributes to the peculiarities of an individual. The parents together contribute one half of the total heritage, the four grandparents together one fourth, the eight great-grandparents one eighth, the sixteen great-great-grandparents one sixteenth and all the remainder of the ancestry one sixteenth.
This law explains another—that the offspring of exceptional parents are, on an average, less exceptional than their parents. Supposing that the average height of two parents exceeds the average height of the race by three inches. The average of the grandparents and remoter ancestors will differ from the average height of the race by much less than this. Since the ancestors beyond the parents contribute one half the entire heritage of the individual, they will act as a drag to pull the individual toward mediocrity, in the present case by one inch. This law acts impartially, so that the offspring of the extremely good and the extremely bad are both saved from the fate of their parents.
This regression toward mediocrity may readily be overcome by selective breeding. In race-horse breeding if the ancestry has been good for three or four generations the rest are not considered.
Galton has devised a forecast machine by which, if the height of the parents is known, the average height to which the offspring will grow may be determined. "Of the individual we can assert nothing as certain, only state the probable."
It is evident from these laws that, if any fond parents feel that they are in any way remarkable, their apparently remarkable offspring are on an average only two thirds as remarkable as they themselves. Also if there are any fond sons or daughters who rely on really gifted parents for their standing among their fellows, the sooner they begin to look to themselves to make up by individual effort their probable loss by the law of regression the better, for, on an average they are but two thirds as gifted as their parents. I should not mention this fool-killing law if it did not have its bright side. If any individual feels it in him to do and be something, a mediocre parentage need not discourage him, for, on the average, exceptional individuals, as we all think ourselves to be, exceed the average of humanity by one half as much again as their parents exceeded this average. (Fig. 3.)
It is a reassuring fact that, starting from any standpoint above the average, our relatives are on the average not quite equal to us. Moreover, a gifted individual is more likely to be the exceptional offspring of mediocre parents than the average offspring of gifted parents. "Among mankind we trust largely for our exceptional men to extreme variations occurring among the commonplace."
What is the physical basis—the vehicle for the transmission of all the characters large and small, from one generation to the next?
In many minute animals and plants where the individual consists of a single cell it simply divides into two, so that it is impossible to say which of the two is the parent, which the offspring. Heredity is here simply a process of growth and division. If one of these individuals is cut into halves each half will regenerate the part lost. Normally the whole animal or plant is concerned in heredity. In fact, an individual may be divided in any way and each fragment will regenerate the lost part so long as the fragment contains part of the nucleus.
In many other animals, of which we nave a representative in our ponds and streams in Hydra, buds develop at certain regions and grow into a new individual. Here any one group out of a large number of groups of cells may build up a new individual. If a hydra be cut in any one of hundreds of possible ways each part will regenerate the portion lost, and so form a new individual. Every group of cells is here adjusted to reproduce the entire individual if the inhibition exercised by the presence of other cells is removed. The method of budding is the commonest means of transmitting the characters in plants. Those individuals produced by buds are usually exactly, or very nearly, like the parents, a fact of which advantage is taken to preserve the peculiar characters of our varieties of fruit, all of which are perpetuated by grafting, or by means of runners.
In some worms the power of developing a new individual if part of the old one has been lost has been modified so that the lost parts are reformed before they are really lost. A given limited part of the middle of the body has the habit of forming a new head for the part behind, and a new tail for the part in front. A string of individuals is formed in this way joined tandem. These separate after some time and each new individual repeats the process.
|Fig. 5.||Fig 6|
|Fig. 5. Hydras: the One at c with Buds; the One at d with Sex-Organs. From Leuckart.|
|Fig. 6. An Arm of a Starfish reproducing Four Lost Arms.|
These methods of forming new individuals are occasional. Each method is restricted to some limited group of species. In practically all animals in which these occasional methods of reproduction occur, they alternate with sexual reproduction. In the great majority of animals sexual reproduction is the only means of transmitting characters to a new generation.
By sexual reproduction we understand the development of a new individual from a single cell which is usually produced by the fusion of two cells.
Just a word as to what we mean by a cell. The word has a penitentiary flavor that may be misleading. A cell is a mass of protoplasm enclosing a differentiated portion or nucleus. The nucleus contains, among other things, during certain phases of cell life a definite number of thread-like bodies called chromosomes. The number of chromosomes differs in different animals, but is always the same in the different cells of the same animal.
In unicellular plants, and in some protozoans, two individuals physically alike, or nearly alike, and moving about in water, unite to form a single individual. In animals more complicated entire individuals no longer merge into one; but this function has become restricted to certain cells, just as the function of moving the animal from place to place has become restricted to certain cells. In the majority of animals living in water these cells are liberated in the water, and here two cells unite, as in lower forms two individuals unite.
The cells uniting are equally important in transmitting hereditary characters, and in their essential structure. As an adaptation to the necessity of the union of two cells and to the necessity that the combined volume of the two cells be sufficient to give the new individual a fair start, the cells, as the result of a division of labor, have become very different in shape and action. One has taken upon itself the function of providing the nutriment necessary to start the new individual,
|Fig. 7.||Fig. 8.|
Fig. 7. Two Typical Cells; Ovarian Eggs of Cymatogaster.
Fig. 8. Photograph of the Conjugation of Two Strings of Individuals of Spirogyra; at a the contents of one individual are passing over into the next; at b the contents of two individuals are united within the wall of one; at c the two uniting individuals have been metamorphosed into a spore; d, a bachelor which missed a mate.
and has become comparatively large and inactive. The other has taken upon itself the function of providing the mobility necessary to insure the union of the two cells. It has become excessively minute, and very mobile. The differences in the cells concerned in sexual reproduction extend to the ducts through which the cells are emitted, and secondarily to the whole individual, so that in every organ, function and psychical trait male and female are different.
Into the question of the advantages of the production of a new individual by the union of two cells we can not enter in detail. Suffice it to say that on the part of some it is looked upon as the union of two hereditary tendencies, which eliminates extreme badness, or what amounts to the same thing, extreme goodness that may be inherent in one of these tendencies, and at the same time insures new combinations of characters from which nature may select the fit. On the part of others it is looked upon as a means of rejuvenescence or union of energies to insure the continuance of life for another span. Loeb has found that an egg that will under all normal conditions develop only after a male cell has entered it, may be caused to develop without the male cell by placing it for a short time in a solution of higher osmotic pressure than that in which it is normally found. It is thus seen that one function of the male cell is either to supply stimulation to the egg to cause it to develop, to regain the lost power of dividing, to rejuvenate it or to act as a catalyzer. But it has long been known that the child may inherit from the father. Indeed, Boveri has shown that the male cell can also develop alone into a new individual if it is supplied with a proper medium of sufficient size.
The egg and the sperm are thus seen to equally contain the hereditary tendencies necessary to form a new individual. Since the offspring frequently resembles both parents this result is evidently caused by the mingling of the two hereditary tendencies.
Boveri's experiment brings us naturally to the question as to where in the hereditary cells the power of reproducing all the complicated transmissible parts lies. In spite of the fact that it is inconceivable that the many hereditary qualities of, say an elephant, should be compressed into two cells, one just large enough to be seen with the unaided eye and the other far too small to be seen without the microscope, he has demonstrated that the hereditary plasma is restricted even to certain parts only of these cells.
The eggs of a sea urchin, Sphærechinus, which normally develop into a well-known larva, were broken by Boveri into two in such a way that the nucleus was all contained in one fragment. Male cells of another species of sea urchin, Echinus, having a well-known, but quite different, larva, he then caused to enter the fragment without a nucleus. A larva developed which possessed all the characters of the larva normal to the male used in the experiment. Since cell contents of both species were present and nuclear structures of only one, and the larva resembled the species represented by a nucleus, it was concluded that the hereditary substance is located in the nucleus. The nucleus, then, contains the physical basis of heredity.
Boveri's results were looked upon with much suspicion until they were confirmed by Delage. With a German and a Frenchman agreeing we may safely consider this point as settled.
The intimate process of the preparation of the hereditary cells for their union and the union of the cells have been the subjects of many monographs during the last twenty years.
Since each cell has a definite number of chromosomes and this number would be doubled by the union of two cells elaborate provisions are made by the cells to reduce this number to one half before the union of the two cells takes place. The study of the methods of this reduction has engaged a host of cytologists during the past ten years, and innumerable papers have resulted.
It is evident that the great difference between the two cells is simply an adaptation to insure their union, for, after uniting, their nuclei, the physical basis of heredity, become alike (c in figure 11).Credit for the great activity in research along this line must be
largely given to Weismann. The method of reduction imagined by him accounted for the origin of variation in the offspring, and thus furnished him with an explanation of the advantages, and therefore the existence of sexual reproduction. It was the attempt to verify or reject his theory which called forth so many monographs on this subject.make it probable that this process is continued with every division, so that ultimately each cell of the adult contains chromosomes, one half of which are the lineal descendants of the chromosomes coming from the father, the other half lineal descendants of the chromosomes coming from the mother. Rückert has found that in a late stage of development in a crustacean the chromosomes were in two groups, presumably maternal and paternal. Moenkhaus has found that in crossing two species of fishes with structurally and physiologically different chromosomes these retained their structural and physiological differences for a number of divisions. From the elaborate provisions to insure the union of the chromatic threads it is quite certain that they, finally, are the carriers of the hereditary power.
The character of any cell is controlled by the nucleus it contains, and, since we have seen that the nucleus contains two different groups of chromosomes, one group containing the peculiarities of the father and the other the peculiarities of the mother, the cause of the blending of the two sets of characters in the offspring becomes apparent and the greater resemblance in some characters to one parent and in other characters to the other parent may readily be inferred.
Without attempting to review the recent speculations and observations on the origin of the hereditary cells, I want to give an outline of some observations I made about ten years ago, and which have recently been confirmed by Beard. Very early in the development of one of the California viviparous fishes certain cells apparently lose their interest in the development. They undergo very little change, while the rest of the cells are busily engaged in multiplying and forming themselves into the various organs of the young fish. These cells become shifted somewhat and probably engage in active migrations. Late in the development they again begin to divide and ultimately give rise to the reproductive cells of the new individual. Not all reach this fate; a certain number are apparently lost in their migrations, and their late history could not be followed. So much seems certain—that none of the cells which became segregated from the rest so early ever became anything but reproductive cells, and that in no case do other cells ever become reproductive cells. The shortest route observed between reproductive cells in the new individual and the egg from which it developed did not exceed fifteen cell divisions. If the divisions had been continuous at the rate of division frequently seen in fish eggs all of them could have taken place in a day or two. Since the last generation of cells are really part of the new generation of individuals the time between the beginning of development and the completion of provisions for the next might have been but one or two days.
Into the question of the origin of heredity and the hereditary power of the reproductive cells I can not go at this time. Suffice it to say that I consider the hereditary power of the reproductive cells the result of a division of labor, just as the high contractile powers of the muscle cells is the result of a division of labor.
It is evident from what has gone before that we can not say that the individual became alive at any given point. Each individual is part and parcel of many individuals who struggled and fought and aspired, who lived, and, above all, succeeded. The fact that he is, is proof positive that, as far as his ancestry is concerned, he deserves to be. He has always been alive since the creation of his remotest ancestor. There has never been a death in the direct line of his ancestry, and he forms a link in a chain that is potentially endless and, in so far he is, through his offspring, physically potentially immortal.
- Photographic illustrations by D. W. Dennis.
- With apologies to Dr. E. G. Conklin.
- Galton in his 'Hereditary Genius' found one hundred gifted men to possess on an average the following number of relatives equally gifted:
Gr.-grandfathers 3 Grandfathers 17 Brothers 41 Fathers 31 Granduncles 5 Nephews 22 Gifted Men 100 Uncles 18 Grandnephews 10 Sons 48 Cousins 13 Grandsons 14 Great-grandsons 3
The isolation of gifted men is graphically illustrated by arranging the facts given in the middle line in a frequency polygon. (Fig. 4.)
- Pearson has demonstrated mathematically by comparing parents with offspring that 'whatever be the physiological function of sex in evolution, it is not the production of greater variability.'
- That the reproductive cells never develop into anything but reproductive cells ought not to continue to confuse us, in the face of the fact that nerve cells, for instance, never give rise to any other kind of cells and other cells are never converted into nerve cells. Even in the regeneration of a lost arm only those sorts of cells are regenerated which have representatives at the cut surface.