Part 5
In experimenting on moths it has been found that the _variations of temperature_ to which the pupæ, and probably also the larvæ, are subjected, tend to bring about very pronounced differences in the moths. Cold has a tendency to develop a darker hue in the perfect insect.
English dogs when taken to hot climates, like that of India, are known to degenerate in a few generations. It is well known how climate affects the hairiness of animals. When greyhounds are taken to the uplands of Mexico they are unable to course on account of the rarity of the air.
In 1870 a number of pupæ of a certain species of moth (Saturnia) were taken from Texas to Switzerland. After passing the winter there, the pupæ emerged from their cocoons as moths, and resembled the Texan species entirely. The young of these moths were fed on the leaves of a plant different from that the moths in Texas feed on, and they developed into moths so different in form and color-pattern from their parents that entomologists classified them as a distinct species.
We have seen how certain foods affect the germ-cells and act as blastogenetic factors; the preceding case and the following show how _certain foods act as somatogenetic factors_ and modify the body-cells. If the bullfinch be fed on hemp seed, its color is changed to black; if the canary be fed on cayenne, its plumage becomes darker; if the common green Amazonian parrot be fed on the fat of siluroid fishes, it assumes a beautiful variegation of red and yellow.
The _character of the soil_ has a marked influence in inducing somatogenetic variations. In France an experimenter collected seed from the wild radish and sowed one lot in heavy soil in the country, while another lot was sown by him in the dry, light soil near the Museum of Natural History in Paris. The radish “roots” grown in these two places presented marked differences in color and form. Those grown in Paris were either of a rose or white color and elongated; while those from the country were violet, dark-brown or nearly black in color, and more rounded than the former.
In the summer of 1847 Professor Buckman gathered seed from wild parsnips, and sowed them in the spring of 1848 under changed conditions of life. Most of the plants grown from these seeds were like the wild parsnips, but some of them developed the light-green color and hairless, smooth appearance characteristic of the cultivated plant. The roots also were found to be more fleshy than those of the wild variety.
Peas and squashes, when grown in different soils, often show remarkable variations.
There is one species of shrimp that inhabits brackish water, and another that lives in water which is much more salt. These crustaceans differ from one another in the character of the spines they bear and in the form of the tail-lobes. They have been regarded as distinct species, and yet either of them can be transformed into the other in the course of a few generations, by gradually altering the saline conditions of the water.
For a long while the siredon and amblystoma were regarded as being distinct genera of amphibians. Siredon was looked upon as a permanent gill-breather, while amblystoma passes through a metamorphosis and becomes a permanent lung-breather. It is now known that the former can change into the latter. If there is plenty of water the siredon remains indefinitely a gill-breather and reproduces freely; but when the water dries up it changes into the lung-breathing amblystoma. These two cases illustrate very well the power of environment to modify the development of organic forms.
As to “_use_” and “_disuse_”: It can readily be observed that exercise increases the size of muscles; that by steady application the capacity for thinking can be developed; that the oarsman’s constant use of his hands leads to the hardening and thickening of the cuticle; that the arm of the blacksmith and the legs of the mountaineer are much enlarged, etc.
When an organ is exercised properly, there is an increased blood supply to it, and, consequently, stimulated nutrition and growth in various parts, such as in the muscular, nervous or other tissues.
When an organ is disused there is diminished blood supply, and, consequently, diminished growth and functional capacity. In man it is known that certain activities, such as coal-heaving, shoemaking, etc., produce recognizable effects upon the muscular system, the skeleton, and other parts of the body.
The peculiar habits of a tribe, such as tree-climbing among those natives of the interior of New Guinea, who build their houses in the upper limbs of lofty trees, modify the body in ways that are readily recognizable.
After considering many facts in connection with the brains of rabbits, Darwin announced that this most complicated and important organ in an animal is subject to the law of decrease in size from disuse. We have very interesting illustrations of the effects of “use” and “disuse” in causing somatogenetic variations, in the differences between domestic ducks and the wild ones from which they have been undoubtedly derived. The wild duck, which must constantly be on the alert for enemies, and uses its wings so much more extensively and its legs comparatively less than the domestic duck, is a much more intelligent fowl than the stupid, well-protected domestic one. The wings of the wild duck are stronger and its legs shorter than those of the barnyard duck. It has been shown that in the wild duck the brain is nearly twice as heavy in proportion to the body as it is in the comparatively imbecile domestic duck.
Many other useful illustrations of disuse, such as the cattle and goats in India, that have dependent ears; also cats in China, and horses in parts of Russia, whose ears are dependent, could be referred to. Use and disuse are included among the factors of environment, because by those terms we mean certain _groups of body-cells that are functionally active or inactive_; for body-cells on any theory of modified pangenesis constitute an exceedingly important environment of the germ-cells.
The surrounding conditions (environment) of an animal or plant having the power to cause variations in the living creature by affecting its germ-cells or its body-cells, the _environment_ may be spoken of as _blastogenetic_ and _somatogenetic_.
Whether it is a fact or not that somatic variations can induce corresponding variations in the germ-cells, and thus be transmitted by heredity, it is certainly true that all heritages must come through the germ-cells. For this reason, it is clearly seen that so far as evolution is concerned the germ-cells are the factors of fundamental importance in organisms. Therefore, we may repeat that environment is the sum-total of the conditions of life that affect the germ-cells directly or indirectly.
ACQUIRED CHARACTERS.
All heritages, then, are derived directly through the germ cells. Can there be any heritages indirectly from the somatic cells through the germ-cells, as has hitherto been assumed? In other words, can acquired characteristics be transmitted to the offspring? This question has given origin to the battle royal that is still going on between opposing schools of biology. The contending parties have appealed to such biological evidence as is furnished by a study of use-inheritance, reflex and instinctive actions in animals, etc., and to such experimental evidence as the induction of traumatic epilepsy in guinea pigs, a change in the shape of the ear by cutting the cervical sympathetic nerve, protrusion of the eyeball through injury to the restiform body of the brain, and such like, noting the effects on the offspring, and have drawn very different conclusions.
As to the transmission or non-transmission of acquired characters, some have maintained that only germinal variations are transmitted (because they believe the germ cells are _insulated_ from the body cells, and therefore from somatic influences). For instance, Ziegler, in his work on _General Pathology_, says: “If a disease, such as nearsightedness, is the product of a special inherited predisposition, _plus_ the effect of harmful influences which have acted upon the body during life, only that part can be transmitted which was received by inheritance, but not that part which was derived from external influences.” In other words, there is no transmission of acquired character. In this belief it will be observed that he follows Weismann.
On the contrary, other investigators, like Darwin and Spencer, teach that somatic variations—the plus element in Ziegler’s illustration of nearsightedness—do influence the germ-cells (through some such agency as Darwin’s theory of pangenesis suggests), and that, therefore, acquired characters can be transmitted. The question is one of fundamental importance, and yet no crucial experiment has been devised or fact observed which can compel the correct answer. The evidence seems to favor the view that acquired characters can be transmitted.
The theories as to the transmission or non-transmission of acquired characters may be better understood by reference to schemes No. 1, 2 and 3. Scheme No. 1 represents the theory of =Pangenesis=, which teaches that reproductive cells are not formed from pre-existing reproductive cells, but by the body cells themselves. Darwin taught that all the cells of the body, such as skeletal-cells, muscle-cells, nerve-cells, and so on, are continually giving off infinitely small cell germs or gemmules, which have the power of growing and forming cells exactly like themselves. These gemmules have a great affinity for one another, and, circulating in the blood in countless numbers, they finally come together in the reproductive glands and form the reproductive cells. On this theory the fact of the transmission of acquired characters can readily be appreciated, and it can easily be understood how the parent molds the child. Suppose, for instance, that the parent, by exercise, has become a skillful athlete. In him certain muscles have become greatly developed and strengthened. During all the time of the exercise of these muscles, the modifying muscle cells have been continually giving off to the blood modified gemmules, which collect in the reproductive cells and make it possible for the offspring to develop into an athlete because the modified gemmules develop into modified muscles like those of the athlete.
Scheme 1 shows the absence of any arrow like those shown in schemes 2 and 3, directly connecting germ-cell with germ-cell; this means that in this theory there is no continuity of the germ-cells. But arrows are seen extending from the various body-cells (skeletal, glandular, etc.) to the germ-cell; this means that the germ-cells are formed by influences or gemmules emanating from the various body cells.
Scheme 2 teaches that a germ-cell (when fertilized, of course) can produce many cells, some of which differentiate, finally, into skeletal cells, some into glandular, some into muscle and nerve cells, and some into new germ-cells; so that an animal or plant, I, is formed. In like manner a germ-cell of animal, I, can give rise to the germ and body cells of animal, II, and so on indefinitely. This scheme shows that there is a direct =continuity of the germ-cells=; and it also shows that the germ-cells are entirely _insulated_, as it were, from the body-cells (skeletal, glandular, etc.), inasmuch as no influences (arrows) extend from the body-cells to the germ-cells. This means that the transmission of acquired characters, bodily, mental, moral, etc., is impossible. It means, in other words, that none of the advantages gained by a parent in the course of his life can be handed on to his offspring by heredity. There are many biologists and pathologists who teach this theory as the correct one.
The majority of biologists accept the theory illustrated in Scheme 3. This is the theory of =modified Pangenesis=, which teaches that there is a direct continuity of the germ-cells and that these germ-cells are not insulated from the body-cells, but that the latter, when modified as the result of experience, can send off influences that correspondingly modify the germ-cells; so that the latter, when developing into a new individual, may cause the same body variations that exist in the parent. In short, this scheme illustrates not only that there is a _germinal inheritance_, but also an _inheritance of acquired characters_. In this Scheme 3, the oblique arrows show that germ-cells produce other germ cells; the perpendicular arrows show that the germ-cells are modified by influences that proceed from the body cells.
_Germinal characteristics are transmitted with vastly greater amplitude and swiftness than merely body (acquired) characteristics._ If, for instance, a man were born with that physical constitution that makes with ease a first-class pianist out of him, his sons may easily, through heredity, be first-class pianists. But if a man be born without such a congenital tendency and has by constant labor and practice so developed the muscles of his forearm, his nerves, his brain, etc., that he becomes a very good pianist (acquired characters); and, further, if his male descendants for thousands of generations, in succession, have become very good pianists by constant practice, we may expect that the sons of these last generations may obtain a congenital tendency to become first-class pianists quite easily. The constant improvement, by practice, of groups of body-cells (muscle-cells, nerve-cells, etc.) for generations, has, in each generation, tended to so correspondingly modify the germ-cells that they have acquired the power to develop into men who may become very good pianists with very little practice. This illustrates that there may be a continuous summation of feeble germ-cell variations that have been induced by prolonged influences emanating from somatic variations, so that, in the course of many generations, robust acquired characters may ultimately be translated into strong congenital characters (Scheme 3).
Scheme 1. =Illustrating the theory of Pangenesis.= Here the germ-cell (a) develops into the body-cells, e, e, e, e, of animal I, as indicated by the oblique arrows, but not into any germ-cells, as indicated by absence of arrow between germ-cell (a) and germ-cells (b). The germ-cells (b) in animal I are formed by the aggregation of infinite numbers of gemmules from the various groups of body-cells, e, e, e, e, as indicated by the perpendicular arrows. The germ-cell (a) transmits germinal heritages to the body-cells e, e, e, e; these body-cells transmit the heritages to the germ-cells (b) by means of the gemmules. If the body-cells are modified in any way, correspondingly modified gemmules are sent to the germ-cells (b), and these germ-cells are modified and thus transmit acquired characters to animal II, and so on.
[Illustration:
I
—> Skeletal-cells (e) (a) Germ-cell —> Glandular-cells (e) —> Muscle-cells (e) —> Nerve-cells (e) | | | | | | | | II | | | | | | | | —> Skeletal-cells (f) v v v v —> Glandular-cells (f) (b) Germ-cell —> Muscle-cells (f) —> Nerve-cells (f) | | | | | | | | III | | | | —> Skeletal-cells (g) v v v v —> Glandular-cells (g) (c) Germ-cell —> Muscle-cells (g) —> Nerve-cells (g) | | | | | | | | v v v v (d) Germ-cell. ]
Scheme 2. =Illustrating the theory of Continuity of the Germ-Cells; pure germinal inheritance; and the non-transmissibility of acquired characters. The germ-cells are insulated from the body cells.= The germ-cell (a) develops into the body-cells, e, e, e, e, and the germ-cells (b), in animal I. The body-cells, e, e, e, e, do not influence in any way the germ-cells (b), as indicated by the absence of perpendicular arrows. The germ-cells (b) get all their heritages from the antecedent germ-cell (a), as indicated by the oblique arrow from (a) to (b). All heritages are purely through the germ-cells. The same with the animals II and III. Germ-cells (a), (b), (c), (d), are connected together by obliquely placed arrows, indicating the continuity of the germ-cells.
[Illustration:
I
—> Skeletal-cells (e) (a) Germ-cell —> Glandular-cells (e) \ —> Muscle-cells (e) \ —> Nerve-cells (e) \ | | | | \ | | | | II \ | | | | \ v v v v —> Skeletal-cells (f) (b) Germ-cell —> Glandular-cells (f) \ —> Muscle-cells (f) \ —> Nerve-cells (f) \ | | | | \ | | | | III \ | | | | \ v v v v —> Skeletal-cells (g) (c) Germ-cell —> Glandular-cells (g) \ —> Muscle-cells (g) \ —> Nerve-cells (g) \ | | | | \ | | | | \ v v v v (d) Germ-cell. ]
Scheme 3. =Illustrating the theory of Continuity with Modified Pangenesis.= A germ-cell (a) develops into the body-cells, e, e, e, e, and the germ-cells (b) of animal I. The germ-cells (b) get their heritages directly from the germ-cell (a), as indicated by the long, obliquely-situated arrow (continuity of the germ-cells). The germ-cells (b) are, moreover, modified by influences extending from the body-cells, e, e, e, e, as indicated by the perpendicular arrows. A modified germ-cell (b) can develop into a modified animal II, and the body-cells of this animal can influence and modify the germ-cells (c); and so on, indefinitely. The perpendicular arrows indicate that acquired characters are transmitted, and that, too, through the germ-cells.
[Illustration:
I
—> Skeletal-cells (e) (a) Germ-cell —> Glandular-cells (e) \ —> Muscle-cells (e) \ —> Nerve-cells (e) \ | | | | \ | | | | II \ | | | | \ | | | | —> Skeletal-cells (f) \ v v v v —> Glandular-cells (f) (b) Germ-cell —> Muscle-cells (f) \ —> Nerve-cells (f) \ | | | | \ | | | | III \ | | | | \ | | | | —> Skeletal-cells (g) \ v v v v —> Glandular-cells (g) (c) Germ-cell —> Muscle-cells (g) \ —> Nerve-cells (g) \ | | | | \ | | | | \ v v v v (d) Germ-cell. ]
Professor Morgan, of England, has advanced the ingenious theory, which may reconcile the above-mentioned antagonistic views, that somatic variations, in the direction of adaptation, pave the way for germinal variations, so that, while somatic modifications _as such_ are not inherited, they are yet the _favoring conditions_ under which germinal variations are preserved by the great principle of natural selection. If this is true, as we think it is, then we can safely state that each man in his totality is the resultant of two great factors—_heredity_ and _environment_, the latter including not only food, water, climate, occupation, etc., but also the character of the civilization, the state of morals in society, the ideals and examples most frequently seen, etc., etc.
Heredity brings down to him the streams of tendency of former generations, often of a healthy and beneficent character, but also often surcharged with lust and passion, and reeking with disease.
Environment is the coöperating and, to us, vitally important factor, inasmuch as it may supplement and thus reënforce the hereditary tendencies, whether good or bad; or it may even tend to turn them into new channels, correcting the evil or vitiating the good.
Man is not simply a creature of the present, but profoundly a product of the past. Bodily structure, moral and intellectual tendencies, disease, vices, and virtues are all in the marvelous stream of heritage that comes to him from the past. “Diseases that no facts in the individual life can account for point gaunt fingers of blame from one generation to another. Not a murderer is hung, not a daughter starts on the downward way, but a great company, like those who were present at the stoning of Stephen, stand by inaugurating and consenting to the ruin.”[4]
Truly has it been said that the past is at work in the present, its powers reaching down through the ages, to all the race, largely molding every human life, touching and influencing every individual’s thought and will, and, more than any other force, coloring history.
Studies in heredity illustrate most luridly that the continuity of the human race is a terrible but remorseless reality.
If the ignorance and the perverted pleasures of one generation may produce the vices and the crimes and the diseases of another, a question of tremendous import arises: Is heredity as potent in the direction of virtue and health as of vice and disease? At the first look one is almost tempted to answer Nay! for the most striking examples of heredity seem to be in the direction of evil. But this is perfectly natural. Decay is always more rapid than growth. A cherry rots much more quickly than it ripens. Vice and disease spread much more quickly and widely than virtue and health. But all history and all social and medical science teach that vice and disease carry within themselves the seeds of decay, and virtue and health the seeds of endurance and growth.
Through the great Darwinian principle of natural selection, or survival of the fittest, vice and disease will become less and less predominant, and virtue and hygienic constitutions more and more disseminated.
As influencing a man’s life and character,[5] which is the stronger factor, heredity or environment? Fatalism or choice? In our opinion, as the result of long study and reading, where we have an average man of “_mens sana in corpore sano_,” environment will be the stronger factor whether for good or for evil—that is, in men in general, who have no organic defect, such as insanity or idiocy, and allied affections, the stronger force is environment; but in those having such defect, heredity is the controlling power, and, we may add, the destroying power.
It must be recalled, though, that the average man with a “sound mind in a sound body,” in his development to his present estate, has become possessed of a vast aggregate of diverse heritages, of varying age, strength and dignity. Some of them are so old and strong that they seem to be cast in unyielding molds, while others are so weak and recent that they fluctuate with every passing circumstance. The most dignified and important of all his heritages is that of rational volition. It is the play of this volition upon many of his other heritages that gives him the power of selecting, to a limited extent, his environment.
Every man is born into the world with a certain physical constitution, and, therefore, with a given temperament; with certain passions; with the power of judgment; and with a certain strength of will. If the power of his will be not equal to the strength of his passions, the latter will surely predominate and will display him as the slave of heredity. If he has such an organization of his nervous system that his volition is superior to his passions, he will be none the less the servant of heredity, though a being now possessed of the power of Free-Will.
Man is, to a far greater degree than is ordinarily realized, the servant of heredity. It seems to us an incontrovertible fact that every living creature, at any given moment, is swayed infinitely more by the _totality of its heritages_ than by its environment. No one can possibly deny this so far as plants and most animals are concerned. Nor, if one look below the surface, can it be denied of the higher animals and of man. Happily, the average man, with his present constitution, has his diverse heritages so proportioned that we may repeat that his life and character (in customs, morals, and religion) are vastly more influenced by environment than by heredity.