Popular Science Monthly/Volume 76/April 1910/Laws of Diminishing Environmental Influence
|LAWS OF DIMINISHING ENVIRONMENTAL INFLUENCE|
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
IT is a widely entertained belief, especially among reformers, philanthropists and many educators, that the force of environment is very great. This view may be the result of vague personal impressions, natural hope, kindliness of heart or perhaps at times professional and selfish interests. But do the facts of science support the expectant hope? Something is needed beyond dogmatic statements and wordy essays.
Experimentally and statistically there is not a grain of proof that ordinarily environment can alter the salient mental and moral traits in any measurable degree from what they were predetermined to be through innate influences. Yet there is naturally a feeling that environment must count for something, and from experimental zoology we know that in many ways its influence is very great. Surely the institutions, discoveries and inventions of civilization form an environment, the value of which from one point of view, is difficult to overestimate. How then can we bring relative order and laws out of the conflicting testimony? It is the purpose of this article by treating the subject from the comparative standpoint, and after a new method, to attempt to harmonize the diversified facts of inheritance and modification.
To distinguish between the relative importance of heredity and environment is not a mere academic question, but a practical one to be answered separately for each biological trait and always with an eye to comparative and proportionate influence. To say that both forces are important is to voice a platitude. To say that they are of equal importance is, in my opinion, to express a falsehood. To say that we can not unravel their interrelations is to turn our back, in a minded way, upon a question of far-reaching consequence. The disputes and confusions which have so long entangled the question I believe to be due to the failure to see certain practical, or common sense aspects—the failure to distinguish between environments which are greatly changed and those which are only slightly altered, between those from which escape is impossible and those from which such escape is relatively easy; the failure to distinguish between environments which are expected and those which are not, and, lastly, perhaps most important of all, the failure to distinguish between effects on higher and on lower types and tissues.
About ten years ago I became convinced through an extended analysis of the genealogical and personal data which exist in histories concerning the royal families of Europe that the influence of environment in creating mental and moral differences among human beings had been greatly overestimated. This conclusion was arrived at from various points of view. Statistical analysis not only supported a theory of germ-cell, or innate causation, but, what was more compelling, an intensive study of each separate family and each isolated group of close relatives brought out such sharp contrasts among the close of kin, such variations in types of mind and character, even when narrowly environed in point of time and place, that a recourse to heredity became forced upon me. The explanation from environment would not work.
This conclusion concerning the relatively slight or unimportant influence of environment as a modifying force on the higher human traits, I announced at the Chicago meeting of the American Psychological Association in 1901; since which time I have been constantly on the lookout for any investigations which might either confirm this belief or necessitate a change of faith. Several direct researches on human heredity have appeared which have been confirmatory in one way or another, and as far as I know nothing has been brought forward to disprove even an extreme belief in the predetermined nature of psychological differences; but it is not the human side of the question that I wish to discuss so much as the results of experimental zoology and botany and their significance to the student of man.
From time to time a vast array of experimental proof has come to my attention showing the profound effect of modification on plants and animals. The word modification I use in its technical sense as proposed by Lloyd Morgan to cover those changes which occur—in the life time of an individual and known to be directly traceable to some natural or artificial (usually artificial) change in its surroundings. These modifications are known to occur and may be easily observed or measured. Whether they are inherited or not is another question, and one entirely outside of the present thesis, for I wish to treat solely of such modifications as are known to occur within the individual life at any time from conception to death or within a single generation. This will throw out of the question a hoard of cases where it is not quite clear whether the observed changes are the result of direct modification, in the true restricted sense, or are perhaps due in part to accumulated influences acting through several generations.
Confining ourselves then to a discussion of modification in the strict sense of the term, let us briefly survey the conclusions of experimental zoology, embryology and botany, and also experiments on regeneration, to see what lesson may be learned from the results of all this painstaking work. It will be of course impossible to treat of more than a fraction of the multitudinous results which have already been handed in as contributions to this young and rapidly growing branch of knowledge—experimental biology; but I believe that even a superficial survey will suffice to bring out my general contention.
The fact that ordinary differences in human environment, as shown in the history of royalty, appeared to alter the innate character and capacity for achievement but little, together with the fact that experimentally a great deal in the way of modification could be produced in the domains of zoology and botany, led me to suppose that there must be some inherent biological differences rendering tissues low in the organic scale especially susceptible to the molding of external influences.
This idea I advanced as an hypothesis in 1906 in the following words:
In 1908 I expressed the same idea in the following passage:
It is to offer the proof for this generalization that I now propose a survey of the whole question.
I wish to state at the start that I have made an effort to collect notes bearing against this theory as well as for it. What I have here to offer is merely a report, almost statistical in its aim and methods, on the general significance of the total number of researches which have thus far been made touching upon modification. Original papers have been occasionally consulted, but the major portion of the notes are drawn from the well-known text-books dealing with these questions and have been rearranged, after a new scheme, under headings expressing in a general way phylogenetic rank. Text-books subdivide the experiments according to the external agents employed, e. g., food, light, heat, gravity, etc., or discuss special types of modification in a disjointed way. The headings that I have made use of are: Plants, Low Metazoa, Mollusks, Crustaceans, Insects, etc., Fishes, Amphibians, Reptiles and Birds and Mammals, and finally Mental and Moral Traits.
It is of course impossible to say at times whether a certain species is higher or lower than another species, but surely we may expect agreement if we say that mammals are higher than birds, that the amniota are higher than the anamniota, and that amphibians are higher than fishes, and that all vertebrates are higher than the low invertebrates. It would be difficult to agree as to the relative rank of the highest of the invertebrates and the lower vertebrates, and difficult to compare the higher plants with low metazoa, but enough of relative rank will be admitted for the purposes of this generalization. Let us now see what chief modifications have been produced (that are nevertheless compatible with life) in these various organic groups.
The modifications in the plant world are so numerous and so striking that one scarcely needs to mention more than a small portion of all the experiments to show that plants are greatly influenced by their surroundings. For instance, plants may be made to grow ten or even twenty times as fast under optimum conditions as compared with their growth under the least favorable. The Japanese dwarf trees show in a remarkable way the possibilities in this direction. The effect is due in part to a mechanical process which prevents the spreading of the branches, but the chief cause is found in poor soil and lack of nourishment.
The great influence of gravity on the direction of the development of different parts of plants is also well known, as is their power of regeneration, where a single begonia leaf may produce a new plant, and even flowers if set in moist sand. The great effect of differences in the amount or quality of heat, light, food, gravity, soil, manure and atmosphere may be easily verified.
Among the low metazoa the influence of temperature, light and food is not so striking as among plants, still the changes may be called remarkable. For instance, the body-length of Echinus larvæ may be made, by raising the temperature, to increase about 25 per cent, of the average growth attained under a low temperature, while the arm-lengths in certain forms are at the same time increased from 200 to 300 per cent. If the number of larvæ of Echinus and Strongylocentrotus, which are growing together, be increased from under 1,500 per liter to over 3,000 per liter the mean length of the anal arms and the oral anus may be made to diminish from the figures 121.2 and 118.4 to 56.6 and 68.5, respectively. No modifications like this have been induced among the higher vertebrates.
In the embryology of placental mammals the dorsal or ventral surface may be facing either up or down, right or left; all forces work out their destiny in disregard to the force of gravity. This force of gravity so important in moulding plants has been proved to be also very influential in the development of hydroids. Pieces of Antennularia antennia produce new stems that grow upward, and stolons that turn downward. Even if the piece is inverted the root arises from the lower end and the stem from the upper. Driesch observed in a species of Sertularia that whenever he altered the position of the piece the new growth changed its position so that the new part turned away from the center of the earth.
The most remarkable of all experiments on low metazoa are the regeneration experiments. Since the exposed cells are subjected to a very considerable alteration of their normal environment, regeneration experiments may be considered as coming under the head of modification experiments. The power of regeneration possessed by worms and hydroids is so well known that only a passing reference is needed here. It is significant that among worms the tail will regenerate more than the head and that the reproductive organs, if removed, never regenerate at all, and the worm remains "incapable of reproducing itself." This last fact is interesting as supporting the view that germ-cells are never reproduced from somatic cells of any kind, much less from the so-called "germinal epithelium," but are the direct descendants of cells that have never lost their germ-cell characteristics.
Artificial parthenogenesis is also easily produced among the low metazoa, and this has been known for a long time. 0. and E. Hertwig published in 1887 their experiments, which showed that various poisons might artificially induce segmentation in the eggs of echinoderms. Loeb carried the process further in 1899 and produced plutei from the unfertilized eggs of the sea urchins by raising the concentration of the sea water. "It was immaterial which substance was used to raise the concentration of the sea water, except for the fact that no substance could be used that injured the eggs too much." Parthenogenesis may be initiated by such a variety of substances as chloroform, urea, sugar, salts and certain acids; but only in low forms of life. At least it is the legitimate inference that it is much easier to produce this effect among the simpler invertebrates. Experiments of this sort on the eggs of fishes are noticeable by their absence. Experiments on the eggs of frogs and Petromyzon have not, as far as I know, resulted in carrying the process beyond the segmentation stage. The experiments on insects which will be discussed under that heading have only a doubtful bearing on the present point, since parthenogenesis is among them more or less of a normal process.
Modification experiments on mollusks have been relatively rare in comparison with the lower invertebrates. This must be, to some extent, due to the practical difficulties which the experimenter would meet on attempting to modify an animal enclosed in a shell.
There is, however, much evidence to show that mollusks may be greatly influenced by their surroundings, at least as regards size. This especially concerns the question of the sizes of snails in relation to the extent of the media in which they are forced to live. The manner in which the volume of water affects the snail's growth is a matter of dispute and does not interest us here. The question for us is, how much are they affected? The answer is, that after about two months the snails grown under optimum environment were more than three times the length, or even five times the length, of those grown under the least favorable conditions. Compare this with similar experiments on plants where the extreme variations artificially induced may be ten or twenty fold, and also with the experiments on the higher vertebrates where extreme variations resulting from any experimentally prescribed conditions are never anything like three to five fold in a linear dimension.
Other classes of modification experiments on mollusks are not so exact, but are perhaps worth mentioning. Vernon writes that "the permanent effects of temperature on size are probably very considerable among many of the mollusks," and quotes Cooke as stating that "a deficiency of lime, in the composition of the soil of any particular locality, produces very marked effects upon the mollusca which inhabit it; they become small and very thin, occasionally almost transparent." Unfortunately there is no statement that changes can be brought about in one generation, though very likely this is the case. Vernon says, in quoting Costa, page 314:
Regeneration experiments are numerous among the mollusks and indicate, as Przibram points out, that these animals belong, with regard to their relation to the question of regeneration, in the middle rank between the lowest invertebrates and the highest vertebrates. He places mollusks in the fourth of six classes with respect to the power of regeneration, which can regenerate besides the tail, also limbs and organs of sense as long as the connection with the central nervous system is intact.
There have not been many modifications experiments performed upon crustaceans except such as come within the more limited field of regeneration. There is evidence, however, that either different quantities of oxygen or differences in amount of the products of metabolism cause marked variation in the rate of reproduction of Daphnia magna. There is also much evidence that among the Daphnias important changes in the life cycle may be artificially induced. These creatures may be made to continue parthenogenetic reproduction into the winter if kept in a warm place.
According to Irvine and Woodhead crabs can not produce their shells if they are allowed to grow in sea water from which chloride of calcium has been excluded, even if sulphate of lime and chloride of sodium are present. Chloride of calcium is absolutely essential for the formation of the crab-shell. This, however, is not the case in the formation of the egg-shell of birds. To quote de Varigny, page 202:
De Varigny then says that other animals offer the reverse instance and alludes to the facts about the crab-shell, but does not see that this apparent contradiction is in any way associated with the great differences in organic complexity between the two animals, or that similar environmental changes have here produced a marked difference in the end product in the case of the lower creature and nothing in the case of the higher.
But these experiments are overshowed, and the great plasticity of this group of invertebrates is chiefly shown by the powers of regeneration which they possess. Experiments showing remarkable regenerative powers of the claws of lobsters, shrimps, crabs, crayfish, etc., are well known, and may be found described at length in any text-book dealing with the subject. In this respect they stand, according to Przibram, about on a level with the mollusks.
As soon as our review brings us to this highly differentiated group of invertebrates we see for the first time the fact that remarkable modifications are now chiefly associated with changes in the integument, especially changes of pigmentation. Profound modifications of a structural sort, and marked differences in body size are no longer met with.
We find, however, that changes in the surrounding temperature, and changes in food, affect the length of time occupied in the different stages of development of Lepidoptera; but in the end the result appears to be the same as under normal conditions. Morgan says: "The moth is identical with the normal," and at the end of his description of these experiments quotes the following: "What the insect gains in the larval stage it loses in the pupa stage."
Some change in the size of the wings of the adult imago is another modification of an unimportant sort, and noted by Standfuss as a result of raising and lowering the external temperature during the larval period.
It will be seen that these variations are not so great as those among plants or among snails.
There is, nevertheless, one very striking and curious kind of permanent modification possible among insects. I refer to the artificial production of royal couples among ants, and also to the production of queens among bees, both brought about by differences in the food supply. It would appear that the factor which determines whether a young female bee becomes a queen or a worker is whether she is, or is not, fed upon the royal food. In the case of ants this is not so clear, for the results may depend largely upon original differences in the eggs. Even if these transformations are brought about solely by environmental alterations they need not have much bearing on the present discussion, since this unusual possibility is of adaptive value to the species and may have been especially evolved and maintained. This is utilized by the insects themselves, "If an old queen dies, or if by accident all the prospective queens have been lost."
Experiments which bring about changes in the life-cycle, and also those which produce parthenogenesis in insects which are normally sexual, can not be considered surprising. As to changes in the lifecycle, the case of the rose aphid is a good illustration. This insect produces parthenogenetically female offspring during the summer months, and normally on the appearance of winter begins to produce both males and females. This winter condition may be indefinitely postponed if the animals are kept in a suitable environment with plenty of food and water. Here we are not witnessing a profound modification, but only the failure of a certain normal change to take place in the failure of its normal stimulus.
Quite separate and different from this is the question of producing segmentation and growth in an unfertilized egg by some artificial chemical or physical means. This is called artificial parthenogenesis, and to have any special bearing on the present discussion must be produced in eggs that normally require fertilization by spermatozoa. The silkworm happened to be one of the first of the invertebrates to lend itself to this form of modification. "In 1886 Tichomiroff published the fact that the unfertilized eggs of the silkworm Bombyx mori, can be caused to develop by rubbing them gently with a brush, or by putting them for a short time into concentrated sulphuric acid."
We can not, however, consider this more than a slight alteration from the normal condition for two reasons. First, because the silkworm is closely allied to forms of moths in which parthenogenesis is normal; and second, because, as Loeb states, "Siebold had already mentioned, and Nussbaum confirmed his observations, that a small number of such eggs develop without these means." This is about the last we shall hear of artificial parthenogenesis, as our review takes us higher and higher in the organic scale of evolution. The results of such attempts as have been made to induce this form of modification among the vertebrates have been very unsatisfactory. Nothing can be made to develop beyond the segmentation stage.
Some experiments like those of Dorfmeister, Weismann, Eimer, Merrifield and Poulton show the direct results of changes in temperature, food and even the color of the surroundings; but these affect the character of the pigmentation in these normally highly pigmented forms, and we shall see, as we go higher in phylogeny, that pigmentation is one of the easiest characters to alter through environmental differences.
Eegeneration experiments are naturally not numerous on the bodies of insects, for practical reasons, but their power to regenerate lost legs and wings falls in line with our generalization. The legs of the lower insects, like the walking-stick and cockroaches, will grow again if amputated, the legs of the higher insects, butterflies, ants, bees and wasps, not so well. Larval insects are placed by Przibram about on a par with adult mollusks, leeches and fishes, and a little behind Amphioxus as far as general power of regeneration is concerned. Insects in the final or adult stage are placed in the highest class along with mammals and birds, but since adult mammals and birds can not regenerate lost limbs it seems as if further subdivision might have been made. Experiments which consist of grafting parts of different species on to one another are possible among insects at least during the pupal stage. But the "integumentary organs" alone show successful union.
There is one way in which insects appear to show less modification than the vertebrates. Removal of the sexual glands from birds and mammals produces, as is well known, certain marked anatomical and physiological changes. These are supposed to be due to the cessation of an internal secretion which the gland normally produces. Removal of the sexual glands from insects does not cause changes in the secondary sexual characters. It seems fair to assume that this is because the sexual glands of insects do not produce any internal secretion. If this is true we should not expect any changes in the other organs of the body, since these other organs have not experienced any change from their normal environment. If, on the other hand, the environment is actually and certainly changed, as is the case when caterpillars are fed on different kinds of food, the variations associated with sex may, to a certain extent, be artificially induced. It is very doubtful if the vertebrates will permit any such modification of sexual differences by changes in their diet.
One of the first points noticed in looking over the results of modification experiments on fishes is that the total number of such experiments is significantly small. There is little to record that can in any way be considered striking or interesting as bearing on the present discussion.
If the under surface of flounders is exposed to light during the growth of the fish the side which is normally white will become pigmented like the upper side, though not so much so. Here we see that what is perhaps the most striking modification produced in a fish is concerned merely in a change of pigmentation, which is probably always one of the easiest changes to bring about.
In hatcheries, it is sometimes desirable to retard the growth of fishes for commercial reasons. The experiments of Meyer, Earll, Rice and others show that lowering the temperature of the water lengthens the interval between fertilization and hatching to about a month. There are no further facts to show whether the fishes are permanently modified thereby. Presumably they are normal in the end, as is the case with retarded insects and frogs. Fishes show good regenerative powers and in accordance with their phylogenetic rank.
Fishes and frogs will not endure the high atmospheric pressure experiments that can be brought to bear upon low invertebrates without loss of life. Although the lower forms which submit to these high pressures (200 to 600 atmospheres) are only temporarily modified and afterwards regain their normal proportions, the facts are significant as showing that higher protoplasm will not submit to the rude and abnormal treatment that the lower will. The higher protoplasm must have its accustomed environment and will not survive if it is ruthlessly forced into very unnatural surroundings.
Within this group one finds modifications brought about by differences in temperature, light, gravity, salt, electricity, atmospheric pressure and food. Experiments with differences of temperature cause merely differences in the rate of growth. These were first performed by Higginbottom some sixty years ago and have been repeated by O. Hertwig and by Lillie and Knowlton. Cold retards the rate of growth considerably; but the point here is, that if these animals continue to develop at all, the adult forms are not essentially different from the normal. Permanent differences in size, such as are produced in snails and especially in plants, are not effected among the amphibians by differences in amount of heat. Extra high temperatures, or those above the optimum, will often produce abnormalities or monstrosities, such as embryos with double heads or tails, but these do not live.
It is also noteworthy that the latitude for possible manipulation, by the use of high temperatures, is not as great here as among plants. Vernon says, page 228:
Light has but little effect upon the growth of amphibians. Gravity, on the other hand, has considerable demonstrable influence on cleavage during the very early stages of the developing egg. This influence of gravity on the growth of amphibians should be contrasted with its far greater effect on plants and hydroids, and also with its probable effect on the developing embryos of mammals, which must here be very slight, if any, judging from the haphazard nature of placental attachments. Gravity may of course play a certain role in mammalian embryology in the very youngest stages. Investigation of this question would in the nature of things be very difficult. But even if it does, its influence in the later stages is certainly very slight and, so far as we can see, negligible. My argument is that the modifying influence of gravity is less in higher organisms than it is in lower, and less in older stages of development than it is in younger.
The effect of changes in salinity and density of the water in which frogs are developing are in general similar to those involving changes in the temperature. Retardation of the rate of growth may be brought about, but the evidence is lacking to show that the animals are any different in the end. It is of interest to note that in this connection there is something that might be thought contradictory to the generalization which I am making. H. de Varigny who has himself experimented upon the effect of introducing common salt into fresh water where tadpoles are growing, states that it is easier to accustom tadpoles of three weeks of age to withstand an excessive amount of salt than it is younger larvae.
But it is not my contention that younger stages of ontogeny may not be more sensitive than older ones to certain abnormal external conditions. A young chick is more sensitive to cold than an older one. My contention is that if we wish to produce a modification, which is nevertheless compatible with life, we can succeed best with the younger stages of development and with lower organisms. In this case no life-compatible modification was produced in either case. The younger tadpoles modified, but died. The older ones did not modify, but continued their development in a normal way. The real reason of the difference may here have been that the vital cells of the older tadpoles were better protected by the outer covering (ectoderm), so that the interchange of fluids (osmosis) worked more gradually in these vital cells. In just the same way the cold injures the young chick more, because from the absence of feathers, the same degree of cold reaches his vital organs more suddenly.
Experiments involving the effects of electricity on the growth of frogs' eggs are not of any special significance. They produce changes in the arrangement of the pigment and sometimes abnormal cleavage, or abnormal development. Differences of atmospheric pressure (presumably really differences in the amount of oxygen absorbed by the water) cause differences in the rate of growth. The researches of Rauber show that
This certainly can not be considered a surprising discovery, nor have the experimenters produced any appreciable modification on amphibians by means of differences in the amount of oxygen.
The development of tadpoles can be considerably retarded by scanty feeding, so that they may be kept in the gill-breathing stage for over a year; but if they survive they still retain their potentialities for becoming normal adults. This is shown by an interesting experiment of de Varigny's. He describes it thus:
Modifications of this sort are shown in the experiments of Kammer. Salamandra atra acquires yellowish white spots through higher temperature and moisture. Low temperature and dry conditions make Salamandra maculosa more black in color with a diminution of the yellow spots.
The amphibians show good powers of regeneration, but their faculty in this direction is neither more nor less than is warranted by their position in the phylogenetic scale. This is well indicated by Przibram. In their younger and lower types they belong in the fourth class of animals, or those which can regenerate not only the tail, but also limbs, sense organs and other portions of the body as long as the central nervous system is not removed. More adult and higher types lose some of this power and then may regenerate the tail only or only certain tissues.
The general conclusion from all these experiments on amphibians involving artificially induced changes in the condition of temperature, light, gravity, salt, electricity, oxygen and food is that when we arrive as high in the phylogenetic scale as the amphibians very little can be done to permanently modify the predetermined forces of the germplasm. As among fishes the most striking permanent changes are concerned with the pigmentation.
In powers of regeneration the reptiles show their higher phylogenetic rank. The lizard can regenerate the tail, but not the limbs. It is interesting that the new tail is not composed of bones, but is a cartilaginous tube attached to the half of the broken seventh caudal vertebra. Snakes and turtles will not regenerate their tails.
Modification experiments, other than those concerning regeneration, are not numerous or suggestive. I find only reference to the well-known pigmental response of the chameleon.
As we ascend the scale we not only find that modification experiments are less striking in character, but also find fewer experiments recorded. Noteworthy modifications of birds are almost entirely concerned with questions of their plumage, and especially with the coloration of the same. A good summary of this knowledge is contained in Vernon.
More remarkable than these observations are the facts ascertained by A. E. Wallace, and communicated by him to Darwin. Thus he states that
Artificially produced alterations in the pigmentation of American birds are shown by the experiments of C. W. Beebe. These experiments demonstrate that the effect of a very humid atmosphere is to increase the dark pigment in the three species studied, namely, the wood thrush, the white-throated sparrow and the inca dove. Beebe mentions that in a state of nature, where the dark forms have been isolated by geographical barriers (and where, of course, natural selection, or other adaptive forces, have been at work for generations), other structural differences are to be found. "With this darkening of the skin structure is frequently correlated a distinction in point of size, either of the body and skeleton as a whole or superficially, as of larger or shorter feathers of the wings or tail." Since Beebe mentions no structural changes of the body as a result of his artificially produced humidity, one infers that the changes were confined to the pigmentations.
In the early stages of embryogeny, heat and light, especially heat, affect the rate of development, but there is nothing, as far as I know, to show that the birds when adults exhibit any variation as a result of all these manipulations.
It is noticeable that the range of temperature (within which any growth is possible) is more restricted for birds than it is for the lower animals.
The comparative difficulty of producing a modification in birds is exemplified, in another interesting way, in the elasticity which the hen possesses of producing a shell when carbonate of lime is absent. This was referred to earlier in this article when commenting on the modification produced in crab-shells.
Regenerative powers, as is well known, are very slight in birds. In two forms at least, the stork and the fighting cock, the beak will regenerate. This fact has been discussed by Weismann and others in connection with the theory of "regeneration and liability to injury," but it does not appear to have been noted that the beak is an integumentary structure and that of all tissues the epidermis is one of the easiest to modify. Neither the wings nor feet of birds will regenerate.
In mammals, as in birds, the chief modifications are concerned with the skin and its appendages.
There is good evidence that changes in climatic surroundings directly affect the color of the hair of some of the mammalia, though at the same time it is evident that others remain unchanged. To a certain extent the white winter coat of the Hudson Bay lemming, and changes in the coloration of hares and rabbits, must be due to direct influence of temperature. Many arctic animals, however, do not change their coat color with the season. Changes in the amount and quality of the hair of various quadrupeds on transportation from one part of the world to another are abundantly recorded.
Other well-known modifications associated with the integument are thickening of the human epidermis by pressure and friction, and darkening of the skin by the action of the sun's rays. The effect of sunlight on the higher animals appears, however, in regard to the vital functions, to be merely superficial. We have, for example, many instances where prisoners have spent long lives in darkness or have perhaps been freed after years of confinement and have then resumed their normal activities. Working mules have been kept in mines for long periods of time, as much as twenty years, " and beyond temporary sensitiveness of the eyes no effect was perceptible."
On the other hand, it is said that arctic explorers experience sluggishness of the mind during the long winter night, as a direct result of the darkness. I do not know how far this is true, but in order to show that this is contradictory to my generalization it would be necessary to prove that the effect is greater upon the minds of men than upon the minds of domestic animals, and greater upon the minds of the leaders of the parties than upon the crews. It would seem improbable that such is the case. Moreover, I am informed by Captain Bartlett of the Roosevelt, that if the men are busy with duties, and if their minds are occupied to the usual extent, no such depressions occur.
With regard to the influences of direct contact on different tissues, I have already noted that pressure produces an easy modification upon the outer skin. Prolonged pressure will also produce noticeable changes in the shapes of growing mammalian bones, but it is probable that even greater modifications might be produced on the skeletons of lower animals. Normally, bone like the epidermis is being constantly remade by proliferation of young cells from the growing layers. In this respect it differs from nerve tissue, the cells of which cease division in early embryonic life.
Boas has recently announced that he has found evidence that the head forms of the children of Hebrew and Sicilian immigrants who come to the United States tend to approach the American type, as a direct result of some mysterious influence of the environment. This he assumes to be of suggestive value to the psychologist and sociologist. He fails to take into account the great anatomical and embryological differences between bone tissue and cerebral nerve tissue. The real deduction from all this work (if indeed it should be confirmed) is that it is easier to modify a bone than it is a brain.
If we consider the effects of different kinds of feeding upon higher animals, as contrasted with the lower, it is evident that the modifications brought about in this way are much less striking among the higher. The linear dimensions of lower organisms may be altered from two to twenty fold. On the other hand, there appears to be an inherent tendency for mammals to grow to a certain definite size within narrow limits. Minot has shown that the rate of growth of guinea-pigs may be artificially altered, but that there is nevertheless a strong tendency for guinea-pigs to grow to a certain size, and that they make up in later stages what they lose in the younger; or if there is an extra increment in the younger stages this is compensated for, later on. This is confirmed by F. B. Sumner for the white mouse. Our general knowledge concerning human twins supports this view. Very frequently twins differ much in size at the time of their birth, even when they are afterwards known as "identical twins" and are difficult to distinguish apart. The puny twin must have been at a disadvantage during uterine life, but this has no permanent effect and is all made up in the end.
It is well known that lecithin given in small quantities in the food will increase the rate of growth of mammals. But it has not been shown, as far as I know, that the ultimate sizes of the adults are thereby made to vary. Let us see what proportionate change in rate of growth has been effected by experiments of this sort.
The communications of Hatai shows that guinea-pigs, rabbits, dogs and rats, after from one to two months' treatment with lecithin, have their rate of growth so altered that the amount of growth is increased from 1.29 to 4.60 times the normal growth during this period. The total weight of the animals is, however, but slightly increased. The animals are merely rendered about two to five per cent, heavier than they normally would have been during the same period of time. Hatai, in referring to the experiments of Danilewski on the eggs of frogs, states that frogs' eggs placed in water containing 1 to 1,500 by weight of lecithin, gained in fifty-four days, 300 per cent, more in weight than those reared in ordinary water.
This would give one the impression that the changes were about the same in ratio for mammals as for tadpoles since the amount of actual additional growth in mammals may be fully three times as much with lecithin as without it. On looking up Danilewski's original paper I find, first that the proportion of lecithin used was not 1 to 1,500 but 1 to 15,000 for the frogs; and second, what is more important, that. the gain of 300 per cent, refers not to the portion gained during the interval when lecithin was given, but to the total weight of the organisms. Thus tadpoles may be made to vary 300 per cent, of their total weight, mammals about two to five per cent. At the same time it seems that the tadpoles are nearly doubled in linear dimensions. It is evident that the mammals are but slightly altered in linear dimensions.
Thus the experimental work of biologists indicates, when we take a comparative bird's-eye view of modification, that environment will be found to be working upon human brain and nerve tissue at its minimum of efficaciousness. Let us see what direct statistical experience has to say on this important problem.
Mental and Moral Traits
The direct researches which have essayed to separate the environment and heredity factors in the higher human traits, and measure approximately their relative influence, are all in substantial agreement. The first light thrown on this question comes from Galton's "History of Twins," published as long ago as 1883. The traits under discussion were physical resemblances, diseases, mannerisms of action, mental disposition, temperament and tastes. The data are not given in completeness, nor in statistical form, but the conclusion seemed to him warranted that, as regards such mental and physical differences as were under discussion, nature prevails strongly over nurture, within the limits which Galton is careful to assign to the latter. This belief was arrived at from a comparison of thirty-five pairs of very similar twins with twenty pairs of dissimilar twins.
Those twins who were similar when young remained so in general, as they grew older; but more significant than this, there appeared to be no tendency for similarities in education and home life to render those originally unlike any more similar with advancing years. The conclusion from Galton's "History of Twins" seemed to be that if the environmental differences are slight no appreciable effect is produced at least upon innate mental differences which are themselves comparatively slight or unimportant, such as differences in tastes, temperament and disposition. This would of course not prove that the more important human differences, such as are represented by success or failure, vices and virtues, are not profoundly modified by environment if the differences in surroundings are considerable.
The history of royalty offers just these remarkably wide differences of an environmental nature. This is somewhat surprising because one might assume that the surroundings would be uniformly superior, as all are of the highest social rank. But for various reasons the individuals have developed under the greatest variety of good and bad influences as regards the atmosphere of their home life, their educational advantages, and opportunities for distinction. Besides, they have lived in different countries and in different eras. Yet, in spite of the fact that the environments show wide variations, these appear to be negligible factors in the production of successful achievement or in the creation of virtuous or vicious types.
That successful achievement is almost entirely due to differences in germ-plasm and is little influenced by environment is the necessary conclusion from the complete analysis of two separate groups of royalty. One of these is the great interrelated group of 3,312 distinct persons in Lehr's Genealogy. This book contains many repeated names, because the same individual appears as an ancestor of different lines, owing to intermarriages. Thus the total number of cases for statistical purposes is much greater than 3,312. It is in fact 32,768, for this book contains eight "families" with 4,096 in each family. Out of the 3,312 different persons there were sixteen who came up to a certain objective standard of distinction and 3,296 failed to do so. The environmental influences must have been mostly distributed at random throughout the group. Yet this did not cause any random distribution of the distinguished persons. Fifteen out of the sixteen were closely related to other distinguished persons.
The second group of royalty contained all the close connections of twenty-three reigning historical dynasties. This group was obtained by a different method, but in part overlaps the other group. Here detailed analysis was made not only of the question of intellectual distinction but of mental and moral variations. Environment was shown to be of little or no consequence in the production of important differences.
The third research to appear on the problem of nature versus nurture is that of E. L. Thorndike, on the origin of mental differences among children attending the public schools in the city of New York. Thorndike, like Galton, used the records of twins to support his argument, but went into the matter with far greater scientific analysis and published all the details of his measurements. He presents:
In concluding his research Thorndike says:
Thorndike's research appears to be very conclusive and confirmatory as far as it goes. Of course one might contend that after all the environmental differences which are experienced by children in the public schools of New York are not very great and that the traits concerned are not really important ones. Such important traits as normal healthy body and mind for a long life of valuable achievement, and a clean bill of character, can only be determined as present or absent, in their varying amounts, after the race of life has been completely or nearly run, and the records of success or failure, of distinction or obscurity, of vices or virtues, have been left behind. The boy is father to the man but our knowledge of biometry already teaches us that this does not mean identity; it merely means a correlation.
The study of children may lead us to wrong conclusions for other reasons. It has already been shown in this article that, other things equal, the young can be more easily affected by surroundings than the adult, and also that there is a great tendency for the higher organisms to equalize in time what they have gained or lost in youth, and to grow after a predetermined plan. For these reasons even the discovery of actual modifications produced among children would not show that the grown men and women, who will be freer to pick and choose their congenial environment, will not follow the same paths that they otherwise would have done.
Pearson and his pupils have recently attempted, by the comparative study of children, to differentiate between the relative influence of heredity and environment. Their results are confirmatory for the special traits studied. In a memoir on vision and sight the authors write as follows, with regard to the effects of environment.
At the close of the paper the authors make the surprising statement that their own research is "the first eugenic study which has endeavored to compare the inheritance and environment factors. We anticipated finding them to be far more comparable in magnitude." If the authors had read a little of the earlier researches on the question of the relative influence of heredity and environment they would neither have spoken of their own eugenic study as the first nor have expressed wonderment at the result.
In "The Relative Strength of Nurture and Nature," Ethel M. Elderton analyzes the above investigation on the eye-sight of children, and also her own study on "The Influence of Parental Occupation and Habit on the Welfare of the Offspring," together with a research of Heron's on "The Influence of Home Environment and Defective Physique on the Intelligence of School Children." These researches "show clearly the small influence of environment." The author on page 28 writes:
That they are contrary to current belief I do not deny, but to say that they are unexpected shows little grasp of the whole biological question of modification or knowledge of results of earlier workers. In fact it will be very surprising if any one succeeds in demonstrating an important environmental control acting on psychological differences, exhibited in mental and moral traits. All the evidence that we possess renders it highly improbable that any of the ordinary differences in human environment, such as riches or poverty, good or bad home life, have more than a very slight effect in modifying these complex and high organic functions the improvement of which is the hope of the altruist and the reformer. Not only do the collected facts indicate as much, but the reasons for the same are not difficult to understand if we consider the laws of diminishing environmental control.
Each organism, whether high or low in the scale of evolution, has from the time of conception and beginning of cell-division and segmentation onward through embryonic and post-embryonic life an expected environment. In other words, it expects to develop and live under conditions which are essentially similar to those which surrounded its immediate ancestors at each stage of their career.
If the expected environment is altered, then the modification which will accrue will in general diminish, (1) in proportion as the change from the expected is less and less in amount. This will follow as a matter of course. Its only interest for us lies in the fact that most alterations in the surroundings that are brought to bear upon human beings are probably not very great in actual differences. They are at least not great in comparison with the experiments of the botanist and zoologist. (2) Environmental influence diminishes with increased phylogenetic rank. (3) Environmental influence diminishes with the evolutionary rank of the tissue affected. (4) Environmental influence diminishes in proportion to the age of the tissue affected. The contents of this paper have been chiefly brought forward to support these second, third and fourth laws. Artificial modification then appears to be easiest upon tissues that are either young or simple, or in a condition of cell subdivision and growth. It must be remembered that the brain-cells, even of a child, are, of all tissues, farthest removed from any of these primordial states. The cells of the brain ceased subdivision long before birth. Therefore, a priori, we must expect relatively little modification of brain function. We next have to consider the question of the possibility of escape on the part of the organism from a novel and perhaps unwelcome environment into its natural one again. (5) Environmental influence diminishes with the organism's power of choice. This may be the chief reason why human beings, who of all creatures have the greatest power to choose the surroundings congenial to their special needs and natures, are so little affected by outward conditions. The occasional able, ambitious and determined member of an obscure or degenerate family can get free from his uncongenial associates. So can the weak or lazy or vicious (even if a black sheep from the finest fold) easily find his natural haunts.
In psychological matters we are dealing with a totally different class of cases from the zoological experiments referred to in this paper. It is a point often forgotten, yet one that should be constantly born in mind, that there are these two kinds of environment from the standpoint of an organism. There are surroundings from which there is no escape, let the creature try his best, and there are also environments from which escape is possible if the inheritant desires impel it. All the modifications on lower animals alluded to in this article are of the first kind, or have been brought about by imposed conditions from which there was no escape. Psychological environment can scarcely be placed in the same category. Therefore the inference is that not only is the brain little influenced by surroundings owing to its high organic rank, but also because most of the varying environments within any one civilization are not absolutely imposed upon the individual, as are experiments upon the lower organisms. This is not meant to imply that differences between one historical age and another, or any other imposed environment from which there is no escape, may not be found of considerable importance in relation to certain sociological and historical facts. For instance, the total number of eminent men in western Europe probably increases too rapidly from the fourteenth century to the sixteenth not to be in part due to the force of circumstances. Also I have statistics in the course of compilation which indicate that there is evidence that women are advancing in noteworthiness of achievement in the United States with each elapsed decade. These imposed and unescapable conditions, which change with the course of history and affect entire races or great groups of people, must be clearly distinguished from the class of environments that exist within any one age and in any one state of civilization.
There are doubtless other ways in which man and other mammals are directly modified by their environment in an essential and lasting way, but to enter into a discussion of these questions is useless in connection with this generalization. Such are the modifications produced by poisons, diseases of a bacterial or other nature, which the individual accidentally encounters. The necessary knowledge has not yet been gained for any generalization, from a comparative point of view, in regard to these complicated processes, so that we should be able to say that these changed conditions affect higher organisms more than lower Moreover, the same poison may be for one kind of protoplasm a great change and for another a slight one, and we have already seen that the proportionate amount of change in the outward conditions is necessarily of prime importance in determining the end result.
These chemical questions do not fall within the literature contained in text-books of experimental zoology, which, to review and rearrange has been the chief purpose of this article. The entire analogy of such experiments, as well as the results of special studies on the relative influence of heredity and environment, can lead to but one conclusion, and that is, that the value of modification diminishes as evolution proceeds.
I know that to generalize is dangerous and exceptions may be found which seem to conflict with the laws or principles which are here set forth, but often apparent exceptions find explanation in the light of further knowledge. I put these laws forth with some hesitancy, yet feel that enough is known to take a step beyond hypotheses and trust that the future will confirm their essential truth.
- "Mental and Moral Heredity in Royalty," New York, Henry Holt & Co., 1906, p. 294.
- Vol. V. American Breeders Association, Report of the Committee on Eugenics, p. 248.
- C. B. Davenport, "Exp. Morph.," pp. 451-452.
- De Varigny, "Exp. Evolution," p. 71.
- Morgan, "Regeneration," p. 74.
- Conf. Morgan, "Exp. Zool.," pp. 44, 265, 266. Vernon, "Variation," pp. 228, 245, 249, 262, 269, 282, 284-286, 312-314. C. B. Davenport, "Exp. Morphology," p. 480. E. Davenport, "Principles of Breeding," pp. 256-264.
- Vernon, "Variation," pp. 229, 296.
- Morgan, "Exp. Zool.," p. 266.
- Morgan, "Regeneration," p. 9.
- For literature, see B. M. Allen, Anatomischer Anzeiger, Band XXIX., 1906, pp. 217-236.
- Loeb, "Dynamics of Living Matter," p. 167.
- Vernon, "Variations," p. 302. C. B. Davenport, "Exp. Morph.," p. 474.
- "Regeneration," Leipzig und Wein, 1909, Tafel XVI. Przibram here, in a chart, shows the general decrease in regenerative power with increase in phylogenetie and ontogenetic stages, but does not treat of other aspects of modification.
- Warren, Q. J. Microsc. Society, Vol. 43, p. 212, 1900.
- "Experimental Zool.," p. 314.
- Vernon, "Variations," p. 230.
- "See Morgan, "Exp. Zool.," pp. 317-320; and Vernon, p. 287.
- Loeb, "Dynamics," p. 165.
- "Experimental Zool.," "Regeneration," Taf. XVI. Die höchsten Formen regeneriern bloss Gewebsdefekte und ungegliederte einfache Hautbildungen (Schnäble, Insektenflügel). For aid in interpreting experiments on insects I am also indebted to Professor W. M. Wheeler.
- Morgan, "Exp. Zool.," p. 301.
- Morgan, "Exp. Zool.," p. 437.
- Vernon, "Variation," p. 25.
- De Varigny, pp. 192-193.
- Morgan, "Exp. Zool.," pp. 262-263. Vernon, p. 249.
- Morgan, "Exp. Zool.," p. 267.
- "Exp. Evolution," pp. 189-190.
- Davenport, "Exp. Morph.," p. 306.
- Arch, für Entwickelungs Mechanik, XVII., pp. 165-264.
- "Regeneration," Leipzig, 1909, Taf. XVI.
- Morgan, "Regeneration," pp. 6, 198.
- "Variation in Animals and Plants," pp. 293-294.
- Amer. Breeders' Assn., Vol. V., 1909, pp. 392-394.
- Morgan, "Exp. Zool.," pp. 261, 262, 459; and Davenport, "Exp. Morph.," p. 459.
- Morgan, "Regeneration," pp. 95, 97, 106.
- Vernon, pp. 243, 330, 331. Morgan, "Exp. Zool.," p. 13.
- De Varigny, pp. 88-91.
- E. Davenport, "Principles of Breeding," p. 244.
- "Senescence and Rejuvenation," Journal of Physiology. Vol. XII., No. 2, 1891.
- Jour, of Experimental Zool., 1909.
- "The Effect of Lecithin on the Growth of the White Rat," Amer. Journal of Physiology, 1904.
- Comptes Rendus, CXXI., 1895.
- "Inquiries into Human Faculty," 1883, pp. 216-243.
- For the arguments which support this belief see Popular Science Monthly, August, 1902-April, 1903 (Vol. LXI., pp. 375, 453, 455, 457. 507. 508; Vol. LXII., pp. 84, 208, 423, 426, 497. 500-503). Same in reprinted form, pp. 9, 17, 19, 21, 2G, 27, 41. 65, 73. 76, 79, 82-85. Additional arguments of a generalized nature may be found in "Mental and Moral Heredity in Royalty: a Statistical Study in History and Psychology," New York, Henry Holt, 1906. pp. 276-298. The arguments drawn from intensive analysis of small groups may be found on pp. 6, 56, 81, 119, 123, 170, 222, 224, 231, 246-247, 248-249. 253-254, 271.
- "Measurements of Twins," Arch. of Philosophy, Psychology and Scientific Methods, New York. The Science Press, 1905, pp. 64.
- Amy Barrington and Karl Pearson, "A First Study of the Inheritance of Vision and the Relative Influence of Heredity and Environment on Sight," London, 1909, pp. 61.
- "Eugenics Laboratory Lecture," Series III., London, Dulau & Co., 1909.
- Since writing this I have received a letter from a distinguished student of heredity containing some remarks on the question of environment versus inheritance. This gentleman, like so many others, does not see that although a result may be due to a complexity of forces we may, nevertheless, measure the relative value of the different components. I refer him, for his encouragement, to the opening chapters of any text-book on physics where the "Laws of Motion," "Parallelogram of Forces" etc., suggest helpful analogies. One illustration which the correspondent gives may serve to make my own standpoint clearer if I answer it here. He says "the question of whether nature or nurture plays the greater part does not arise. As well might we ask whether the locomotive or the steam plays the greater part in transporting the train." My answer is, that by all the needs of a suffering humanity or the development of a rational understanding of the present, past or future of this same species, Homo sapiens, the question does arise; and second that the illustration from the locomotive will do as well as any other. Here, it is the locomotive, not the steam, which is the essential thing. The purchasing agent pays the highest prices for the very best machine because he knows that having got his best machine he can easily get his fuel and his water. He expects to get these. These form the expected environment which may differ some in quality and effect, but after all, from the practical standpoint, the essential thing is the quality of the machine. It is just so with the human mind. Nature is the great decider because nurture is expected.
- This would be the conclusion from Cattell's "Statistical Study of Eminent Men," Popular Science, February, 1903, and Ellis's "Study of British Genius," p. 12.
15 Morgan, "Exp. Zool.," pp. 312, 336.