ADAPTATION TO ENVIRONMENT

1. It is assumed by certain biologists that the environ­ment influences the organism in such a way as to increase its adapta­tion. Were this correct it would not contradict a purely physico­chemical concep­tion of life; it would only call for an explana­tion of the mechanism by which the adapta­tion is brought about. There are striking cases on record which warn us against the universal correctness of the view that the environ­ment causes an adaptive modifica­tion of the organism. Thus the writer pointed out in 1889 that positive helio­tropism occurs in organisms which have no opportunity to make use of it,[275] e. g., Cuma rathkii, a crustacean living in the mud, and the caterpillars of the willow borer living under the bark of the trees. We understand today why this should be so, since helio­tropism depends upon the presence of photo­sensitive substances, and it can readily be seen that the ques­tion of use or disuse has nothing to do with the produc­tion of certain harmless chemical compounds in the body. A much more striking example is offered in the case of galvano­tropism. Many organisms show the phenomenon of galvano­tropism, yet, as the writer pointed out years ago, galvano­tropism is purely a laboratory product and no animal has ever had a chance or will ever have a chance to be exposed to a constant current except in the laboratory of a scientist. This fact is as much of a puzzle to the selec­tionist and to the Lamarckian (who would be at a loss to explain how outside condi­tions could have developed this tropism) as to the vitalist who would have to admit that the genes and supergenes indulge occasionally in queer freaks and lapses. The only consistent attitude is that of the physicist who assumes that the reac­tions and structures of animals are consequences of the chemical and physical forces, which no more serve a purpose than those forces responsible for the solar systems. From this viewpoint it is comprehensible why utterly useless tropisms or structures should occur in organisms.

2. A famous case for the apparent adapta­tion of animals to environ­ment has been the blind cave animals. It is known that in caves blind salamanders, blind fishes, and blind insects are common, while such forms are comparatively rare in the open. This fact has suggested the idea that the darkness of the cave was the cause of the degenera­tion of the eyes. A closer investiga­tion leads, however, to a different explana­tion. Eigenmann has shown that of the species of salamanders living habitually in North American caves, two have apparently quite normal eyes. They are Spelerpes maculicauda and Spelerpes stejnegeri. Two others living in caves have quite degenerate eyes, Typhlotriton spelæus and Typhlomolge rathbuni. If disuse is the direct cause of blindness we must inquire why Spelerpes is not blind.

Another difficulty arises from the fact that a blind fish Typhlogobius is found in the open (on the coast of southern California) in shallow water, where it lives under rocks in holes occupied by shrimps. The ques­tion must again be raised: How can it happen that in spite of exposure to light Typhlogobius is blind?

The most important fact is perhaps the one found by Eigenmann in the fishes of the family of Amblyopsidæ. Six species of this group live permanently in caves, are not found in the open, and have abnormal eyes, while one lives permanently in the open, is never found in caves, and one comes from subterranean springs. The one form which is found only in the open, Chologaster cornutus, has a simplified retina as well as a comparatively small eye, in other words, its eye is not normal. This indicates the possibility that the other representatives which are found only in caves also might have abnormal eyes even if they had never lived in caves.

Through these facts the old idea becomes ques­tionable, namely, that the cave animals had originally been animals with normal eyes which owing to disuse had undergone a gradual hereditary degenera­tion.

Recent experi­ments made on the embryos of the fish Fundulus have yielded the result that it is possible to produce blindness in fish by various means other than lack of light.[276] Thus the writer found that by crossing the egg of Fundulus with the sperm of a widely different species, namely, Menidia, blind embryos were produced very frequently; that is to say such embryos had the degenerate eyes characteristic of blind cave fishes. Very often no other external trace of an eye, except a gathering of pigment, could be found, while a close histological examina­tion would possibly have resulted in the demonstra­tion of rudiments of a lens and other tissues of the eye.

Another method of producing blind fish embryos consists in exposing the egg immediately, or soon after fertiliza­tion, to a temperature between 0° and 2° C. for a number of hours. Many embryos are killed by this treatment, but those which survive behave very much like the hybrids between Fundulus and Menidia, i. e., a number of them have quite degenerated eyes. If the eggs have once formed an embryo they can be kept at the temperature of 0° for a month or more without giving rise to blind animals. Occasionally such rudimentary eyes were also observed when eggs were kept in a solu­tion containing a trace of KCN. Stockard has succeeded in producing cyclopean eyes in Fundulus by adding an excess of magnesium salt to the sea water in which the eggs developed or by adding alcohol, and McClendon has confirmed and added to these results.

The writer tried repeatedly, but in vain, to produce Fundulus with deficient eyes by keeping the embryos in the dark. Sperm and egg were not allowed to be exposed to the light yet the embryos without excep­tion had normal eyes.

F. Payne raised sixty-nine successive genera­tions of a fly Drosophila in the dark, but the eyes and the reac­tion of the insects to light remained perfectly normal.

Uhlenhuth has recently demonstrated in a very striking way that the development of the eyes does not depend upon the influence of light or upon the eyes func­tioning. He transplanted the eyes of young salamanders into different parts of their bodies where they were no longer connected with the optic nerves. The eyes after transplanta­tion underwent a degenera­tion which was followed by a complete regenera­tion. He showed that this regenera­tion took place in complete darkness and that the transplanted eyes remained normal in salamanders kept in the dark for fifteen months. Hence the eyes which were no longer in connec­tion with the central nervous system, which had received no light, and could not have func­tioned, regenerated and remained normal. The degenera­tion which took place in the eyes immediately after being transplanted was apparently due to the interrup­tion of the circula­tion in the eye, and the regenera­tion commenced in all probability with the re-establishment of the circula­tion in the transplanted organ.

In our own experi­ments it can be shown that the circula­tion in the embryo was deficient in all cases where the eyes degenerated. The hybrids between Fundulus and Menidia have often a beating heart but rarely a circula­tion (although they form blood); and the same phenomenon occurred in the embryos which were exposed to a low temperature at an early period of their lives. Hence all the facts agree that condi­tions which lead to an abnormal circula­tion (and consequently also to an abnormal or inadequate nutri­tion of the embryonic eye) may prevent development and lead to the forma­tion of blind fishes. Eigenmann states that no blood-vessels enter the eye of the blind cave salamander Typhlotriton. The presence or absence of light does not usually interfere with the circula­tion or nutri­tion of the embryonic eye, and hence does not as a rule lead to the forma­tion of degenerated eyes.

This would lead us to the assump­tion that the blind fish owe their deficiency not to lack of light but to a condi­tion which interferes with the circula­tion in the embryonic eye. Such a condi­tion might be brought about by an anomaly in the germ plasm or in one chromo­some, the nature and cause of which we are not able to determine at present; but which, since it occurs in the germ plasm or the chromo­somes, must be hereditary. This would explain why it is, that animals with perfect eyes may occur in caves and why perfectly blind animals may occur in the open. It leaves, however, one point unexplained; namely, the greater frequency of blind species in caves or in the dark and the relative scarcity of such forms in the open.

Eigenmann has shown that all those forms which live in caves were adapted to life in the dark before they entered the cave.[277] These animals are all negatively helio­tropic and positively stereotropic, and with these tropisms they would be forced to enter a cave whenever they are put at the entrance. Even those among the Amblyopsidæ which live in the open have the tropisms of the cave dweller. This eliminates the idea that the cave adapted the animals for the life in the dark.

Only those animals can thrive in caves which for their feeding and mating do not depend upon visual mechanisms; and conversely, animals which are not provided with visual mechanisms can hold their own in the open, where they meet the competi­tion of animals which can see, only under excep­tional condi­tions. This seems to account for the fact that in caves blind species are comparatively more prevalent than in the open.

In other words, the adapta­tion of blind animals to the cave is only apparent; they were adapted to cave life before they entered the cave. Many animals are obviously burdened with a germinal abnormality giving rise to imperfec­tion and smallness of the eye—the hereditary factor involved may have to do with the development of the blood-vessels and lymphatics of the eye. Such mutants can survive more easily in the cave, where they do not have to meet the competi­tion of seeing forms, than in the open. In man also an hereditary form of blindness is known, the so-called hereditary glaucoma. It has nothing to do with light, but the disease seems to be due to an hereditary anomaly of the circula­tion in the eye.

Kammerer[278] has recently reported that by keeping the blind European cave salamander Proteus anguinus under certain condi­tions of illumina­tion he succeeded in producing two specimens with larger eyes. According to him the eyes of Proteus may develop to a certain point and then retrogress again. He states that by keeping young salamanders alternately for a week or two in sunlight and in a dark room where they were exposed to red incandescent light, two males formed somewhat larger eyes. The first year no altera­tion was visible. In the second year a slight increase in the size of the eyes was noticeable under the skin. In the third year the eye protruded slightly and this increased somewhat in the fourth year.

There is thus far only one case on record in animal biology in which the light influences the forma­tion of organs. The writer found that the regenera­tion of the polyps of the hydroid Eudendrium does not take place if the animals are kept in the dark, while the polyps will regenerate if exposed to the light;[279] and the time of exposure may be rather short according to Goldfarb.[280] It is possible that Proteus resembles in this respect Eudendrium; it should be stated, however, that of many different forms tried by the writer over a number of years, Eudendrium was the only one which gave evidence of such an influence of light. Of course it is not impossible that the light might influence reflexly the development of blood-vessels in the eyes of certain animals, e. g., Proteus, and thus allow the eyes of Proteus to grow a little larger.

We therefore come to the conclusion that it is not the cave that made animals blind but that animals with a hereditary tendency towards a degenera­tion of the eyes can survive in a cave while they can only excep­tionally survive in the open. The cause of the degenera­tion is a disturbance in the circula­tion and nutri­tion of the eye, which is as a rule independent of the presence or absence of light.

We may by way of a digression stop for a moment to consider the most astonishing and uncanny case of adapta­tion; namely, the forma­tion of the transparent refractive media, especially the lens in front of the retina. It is due to these media that the rays which are sent out by a luminous point can be united to an image point on the retina. One part of this process is understood; namely, the forma­tion of a lens. Wherever the optic cup of the embryo is transplanted under the epithelium the latter will be trans­formed into a transparent lens. When the upper edge of the iris is injured in the salamander so that the cells can multiply, the mass of newly formed cells also becomes transparent and a lens is formed. This indicates the existence of a substance in the optic cup which makes the epithelial cells transparent; and which also limits the size of the lens which is formed. The lens is not always a perfect optical instrument, on the contrary, it is as a rule somewhat defective. Of course, a great many details concerning the process of lens regenera­tion have still to be worked out.

3. It is well known that most marine animals die if put into fresh water and vice versa; and in salt lakes or ponds with a concentra­tion of salt so high that most marine animals would succumb if suddenly transferred to such a solu­tion we have a limited fauna and flora. The common idea is that marine animals become adapted to fresh water or vice versa; or to the condi­tions in salt lakes; especially if the changes take place gradually. Yet it can be shown that the existence of these different faunas can be explained without the assump­tion of an adaptive effect of the environ­ment. The writer has worked with a marine fish Fundulus whose eggs develop naturally in sea water which, however, will develop just as well in distilled water; and the young fish hatching in distilled water live and grow in this medium. Most of the adult fish die after several days, when put suddenly into distilled water, but they can live in fresh water which contains only a trace of salt. They can also live in very concentrated sea water, e. g., twice the normal concentra­tion. Suppose that a bay of the ocean containing such fish should suddenly become landlocked and the concentra­tion of the sea water be thus raised to twice its natural amount; the majority of forms would die and only Fundulus and possibly a few other species with the same degree of resistance would survive. An investigator examining the salinity of the water and not knowing the natural resistance of Fundulus to changes in concentra­tion would be inclined to assume that he had before him an instance of a gradual adapta­tion of the fish to a higher concentra­tion of the sea water; whereas the fish was already immune to this high concentra­tion before coming in contact with it.

This fish seemed a favourable object from which to find out how far an adapta­tion to the environ­ment really existed; and the result was surprising. By changing the concentra­tion of the sea water gradually it is possible to raise the natural resistance of the fish only a trifle, not much over ten per cent. The concentra­tion of the natural sea water is a little over that of a m/2 solu­tion of NaCl+KCl+CaCl2 in the propor­tion in which these three salts exist in the sea water. When adult Fundulus are put into a 10⁄8 m solu­tion of NaCl+KCl+CaCl2 in the propor­tion in which these salts occur in sea water they die in less than a day, but when put from sea water directly into a 8⁄8 m or 9⁄8 m solu­tion they can live indefinitely. It was found[281] that if the concentra­tion of the sea water was raised gradually (by m/8 a day) the fish on the fifth day could resist a 10⁄8 m solu­tion of NaCl+KCl+CaCl2 for a month (or possibly indefinitely; the experi­ment was discontinued after that period). When a 10⁄8 m solu­tion was allowed to become more concentrated slowly by evapora­tion (at room temperature) all the fish died rapidly when the concentra­tion was 12⁄8 m or even below. In higher concentra­tions they can live only a day or two. These experi­ments show that while the fish is naturally immune to a 9⁄8 m NaCl+KCl+CaCl2 solu­tion, by the method of slowly raising the concentra­tion it may be made to tolerate a 10⁄8 m or 11⁄8 m solu­tion, but not more. These fish when once adapted to a 10⁄8 m solu­tion can be put suddenly into a very weak solu­tion, e. g., a m/80 NaCl, without suffering and when brought back into a 10⁄8 m solu­tion of NaCl+KCl+CaCl2 they will continue to live. If they remain for several days in the weak solu­tion their power of resistance to 10⁄8 m NaCl+KCl+CaCl2 solu­tion is weakened.

What change takes place when the fish is made more resistant and why is its normal resistance so great? The answer based on the writer’s experi­ments seems to be as follows: Fundulus is comparatively resistant to sudden changes in the concentra­tion of the sea water between m/80 and 9⁄8 m because it possesses a comparatively impermeable skin whose permeability is not seriously altered by sudden changes within these limits of concentra­tion; while if these limits are exceeded and the fish are brought suddenly into too high a concentra­tion the skin becomes permeable and the fish dies, the gills becoming unfit for use or nerves being injured by the salt which diffuses into the fish.

The fact, that by slowly raising the concentra­tion to 10⁄8 m the fish may resist this limit, is in reality no adapta­tion. There is no sharp limit between the injurious and non-injurious concentra­tion. We have seen that the fish is naturally immune to a 9⁄8 m solu­tion. It is also naturally immune to a 10⁄8 m or 11⁄8 m solu­tion if we give it time to compensate the injurious effects of a 10⁄8 m solu­tion by the repairing action of its blood or kidneys. Beyond this no rise is possible. In reality adapta­tion does not exist in this case.

In former experiments the writer had shown that a pure NaCl solu­tion of that concentra­tion in which this fish naturally lives kills it very rapidly, while it lives in such a solu­tion indefinitely if a little CaCl2 is added. The explana­tion of this fact is that the pure NaCl solu­tion is able to diffuse into the tissues of the animal while the addi­tion of a trace of CaCl2 renders the membrane practically impermeable to NaCl. The ques­tion then arose whether it was possible to make the fish more resistant to a pure NaCl solu­tion of sufficiently high concentra­tion and how this could be done. On the basis of the idea of an adaptive effect of the environ­ment we should expect that by gradually raising the concentra­tion of a pure NaCl solu­tion the latter would gradually alter the animal and make it more resistant. The method of procedure suggested was therefore to put the fish first in low and gradually into increasing concentra­tions of NaCl. This method was tried and found futile for the purpose. Fundulus when put from sea water (after having been washed) into a 6⁄8 m NaCl solu­tion die in about four hours. When kept previously in a weaker NaCl solu­tion they die if anything more quickly. But it is possible to make them live longer in a 6⁄8 m solu­tion of NaCl; we have to proceed, however, by a method which is in contrast with the ideas of the adaptive influence of the environ­ment. When the fish are first treated with sea water (or with a mixture of NaCl+KCl+CaCl2) of a higher concentra­tion so that they become adapted to a 10⁄8 m solu­tion of NaCl+KCl+CaCl2 or to 10⁄8 m sea water, they become also more resistant to an otherwise toxic solu­tion of NaCl. Fish taken directly from sea water were killed in less than four hours when put into a 6⁄8 m NaCl solu­tion, while fish of the same lot previously adapted to 10⁄8 m sea water in the manner described above lived two or three days in a 6⁄8 m NaCl solu­tion.[282]

It is not impossible that it was the high concentra­tion of calcium in the 10⁄8 m sea water which rendered the fish more immune to a subsequent treatment with NaCl. We know why a pure NaCl solu­tion kills them and we also know why the addi­tion of CaCl2 protects them against this pernicious effect. It is rather strange that where the condi­tions of the experi­ments are clear we find nothing to indicate an adaptive effect of the environ­ment.

4. Ehrlich’s work on trypanosomes seems to indicate a remarkable power of adapta­tion on the part of organisms to certain poisons. If the writer understands these experi­ments correctly they consisted in infecting a mouse with a certain strain of trypanosomes, and treating it with a certain arsenic compound, which inhibited somewhat the propaga­tion of the parasites but did not kill them all. Four or five days later trypanosomes from this mouse were transmitted to another mouse and after twenty-four hours this mouse was treated with a stronger dose of the same arsenic compound; and this process was repeated. After the third transmission or later, the trypanosomes can resist considerably higher doses of the same poison than at first and this resistance is retained for years. Ehrlich seems to have taken it for granted that he had succeeded in trans­forming the surviving trypanosomes into a type which is permanently more resistant to the arsenic compound than was the original strain.

The writer is not entirely convinced that in these experi­ments a possibility was sufficiently considered which is suggested by Johannsen’s experi­ments on the importance of pure lines in work on heredity. According to this author a strain of trypanosomes taken at random should, in all likelihood, contain a popula­tion consisting of strains with different degrees of resistance. If a high but not the maximal concentra­tion of an arsenic compound is repeatedly injected into the infected mice the weaker popula­tions of trypanosomes are killed and only the more resistant survive. These of course continue to retain their resistance if transplanted to hosts of the same species. According to this interpreta­tion the arsenic-fast strain may possibly have existed before the experi­ments were made, and Ehrlich’s treatment consisted only in eliminating the less resistant strains.

On the other hand, it has been shown that if an arsenic-fast strain of trypanosomes is carried through a tsetse fly it loses its arsenic-fastness. This fact may possibly eliminate the applicability of the pure line theory to a discussion of the nature of the arsenic-fastness, but it seems that further experi­ments are desirable.

5. Dallinger stated that he succeeded in adapting certain protozoans to a temperature of 70° C. by gradually raising their temperature during several years. It is desirable that this statement be verified; until this is done doubts are justified. Schottelius found that colonies of Micrococcus prodigiosus when transferred from a temperature of 22° to that of 38° no longer formed pigment and trimethylamine. After the cocci had been cultivated for ten or fifteen genera­tions at 38° they failed to form pigment even when transferred back to 22° C. Dieudonné[283] used Bacillus fluorescens for similar purposes. At 22° it forms a fluorescing pigment and trimethylamine, but not at 35°. By constantly cultivating this bacillus at 35° Dieudonné found that after the fifteenth genera­tion had been cultivated at 35° the bacillus produced pigment and trimethylamine at 35°. Davenport and Castle[284] found that tadpoles of a frog kept at 15° went into heat rigour at 40.3° C., while those kept for twenty-eight days at 25° were not affected by this temperature but went into heat rigour at 43.5°. When the latter tadpoles were put back for seventeen days to a temperature of 15° they had lost their resistance to high temperature partially, but not completely, since they went into heat rigour at 41.6°. The authors suggest that this adapta­tion to a higher temperature is due to a loss of water on the part of protoplasm, whereby the latter becomes more resistant to an increase in temperature. This idea was put to a test by Kryž[285], who found that the coagula­tion temperature of their muscle plasm is not altered by keeping cold-blooded animals at different temperatures.

Loeb and Wasteneys[286] found that Fundulus taken from a low temperature of 10° C. die in less than two hours when suddenly transferred to sea water of 29° C.; and in a few minutes if suddenly transferred to a temperature of 35° C. If, however, the fish were transferred to a temperature of 27° C. for forty hours they could live indefinitely in sea water of 35°. By exposing the fish each day two hours to a gradually rising temperature they could render them resistant to a temperature of 39°. The remarkable fact was that fish if once made resistant to a high temperature (35°) did not lose this resistance when kept for four weeks at from 10° to 14° C. Control fish taken from the same temperature died in from two to four minutes; immunized fish taken from 10° and put directly to 35° C. lived for many hours or indefinitely. They will even retain this immunity when kept for two weeks at a temperature of 0.4° C.

Why is it that an animal can in general resist a high temperature better if the latter is raised gradually than when it is raised suddenly? Physics offers us an analogy to this phenomenon in the experience that glass vessels which burst easily when their temperature is raised suddenly, remain intact when the temperature is raised gradually. Glass is a poor conductor of heat and when the temperature is raised suddenly inside a glass cylinder the inner layer of the cylinder expands while the outer layer on account of the slowness of conduc­tion of heat does not expand equally and the cylinder may burst. We might assume that the sudden increase in temperature brings about certain changes in the cells (e. g., an increase in permeability or destruc­tion of the surface layer?). If the rise of temperature occurs gradually the blood or lymph or the cell sap may have time to repair the damage, and this repair seems to be irreversible, at least for some time, as the experi­ments on Fundulus seem to indicate. If the temperature rises too rapidly the damage cannot be repaired quickly enough by the cell or body liquids.

It is also to be considered that substances might be formed in the body at a higher temperature which do not exist at a lower temperature, and vice versa, and this might explain results like those of Schottelius or Dieudonné and many others.

6. The theory of an adapting effect of the environ­ment has often been linked with the assump­tion of the inheritance of acquired characters. The older claims of the hereditary transmission of acquired characters, such as Brown-Séquard’s epilepsy in guinea pigs after the cutting of the sciatic nerve, have been shown to be unjustified or have found a different and more rational explana­tion. Recently P. Kammerer has claimed to have proven by new experi­ments that by environ­mental changes, hereditary changes can be produced.

It has been mentioned already that the mature male frogs and toads possess during the breeding season lumps on the thumbs or arms which are pigmented and which bear numerous minute horny black spines; these secondary sexual characters serve the male frog in holding the females in the water during copula­tion. There is one species which does not possess this sexual character, namely the male of the so-called midwife toad (Alytes obstetricans). In this species the animals copulate on land, and it is natural to connect the lack of this secondary sexual character in the male with its different breeding habit. Kammerer now forced such toads to copulate in water instead of on land (by keeping the animals in a terrarium with a high temperature). He makes the statement that by forcing the parents to lay their eggs during successive spawning periods in water he finally obtained offspring which under normal temperature condi­tions lay their eggs naturally in water; in other words, they have changed their habits. We will not discuss this part of his statement since the breeding habits of animals in captivity are liable to be abnormal. But Kammerer makes the further important statement[287] that the male offspring of such couples will in the third genera­tion produce the swelling on the thumb and the usual roughness, and in the fourth genera­tion black pads and hypertrophy of the muscles of the forearm will appear. In other words, he reports having succeeded in producing an inheritance of an acquired morpho­logical character which has never been known to occur in this species. Bateson, on account of the importance of the case, wished to examine it more closely and I will quote his report.

The systematists who have made a special study of Batrachia appear to be agreed that Alytes in nature does not have these structures; and when individuals possessing them can be produced for inspec­tion it will, I think, be time to examine the evidence for the inheritance of acquired characters more seriously. I wrote to Dr. Kammerer in July, 1910, asking him for the loan of such a specimen and on visiting the Biologische Versuchsanstalt in September of the same year I made the same request, but hitherto none has been produced. In matters of this kind much generally depends on interpreta­tions made at the time of observa­tion; here, however, is an example which could readily be attested by preserved material.[288]

More recently the same author has reported another hereditary morpho­logical change brought about by outside condi­tions.[289] A certain salamander (Salamandra maculosa) has yellow spots on a generally dark skin. Kammerer states that if such salamanders are kept on a yellow ground they become more yellow, not by an extension of the chromatophores (which would not be surprising) but by actual multiplica­tion and growth of the yellow pigment cells; while the black skin is inhibited in its growth. The reverse is true if these salamanders are kept on black soil; in this case according to Kammerer the growth of the yellow cells of the skin is inhibited while the black part of the skin grows. Curiously enough, according to him, these induced changes are hereditary. Here again we are dealing with the inheritance of an acquired morpho­logical character.

Megusar[290] has repeated Kammerer’s experi­ments on salamanders but contradicts him by stating that the colour of the soil has no influence on the coloura­tion of salamanders. Of course, we know the phenomenon of colour adapta­tion in which the animal changes its colour pattern according to the environ­ment. This is an effect of the retina image on the skin and has been interpreted by the writer as a case of colour telephotography, for which no physical explana­tion has yet been found.[291] This phenomenon, however, does not lead to any hereditary change of colour.

Kammerer makes many statements on the heredity of acquired modifica­tions of instinct; indeed he claims that an interest in music on the part of parents produces offspring with musical talent. In such claims much depends upon the subjective interpreta­tion of the observer.

The writer is not aware that there is at present on record a single adequate proof of the heredity of an acquired character. We have records of changes in the offspring by poisoning the germ plasm by alcohol given to parents—as in Stockard’s well-known experi­ments—or by exposing butterflies to extreme temperatures, but in these cases the germ cells were poisoned or altered by the alcohol or by chemical compounds produced at very low or very high temperatures. This is of course an entirely different thing from stating that by inducing the midwife toad to lay its eggs in the water the male offspring acquires the pads and horns of other species of frogs on its thumb; or that by keeping black salamanders on yellow paper the offspring is more yellow. Yet if there is an inheritance of acquired characters which can in any way throw light on the so-called phenomena of adapta­tion it must consist in results such as Kammerer claims to have obtained.

While the writer does not decline to accept Ehrlich’s interpreta­tion of the arsenic-fast strains of trypanosomes or Kammerer’s statements in regard to the inheritance of acquired character, he feels that more work should be done before they can be used for our problem.

7. This attitude leaves us in a quandary. The whole animated world is seemingly a symphony of adapta­tion. We have mentioned already the eye with its refractive media so well curved and placed that a more or less perfect image of the outside objects is focussed exactly on the retina; and this in spite of the fact that lens and retina develop independently; we have mentioned and discussed the cases of instincts or automatic arrangements which are required to perpetuate life—the attrac­tion of the two sexes and the automatic mechanisms by which sperm and egg are brought together; the maternal instincts by which the young are taken care of; and all those adapta­tions by which animals get their food and the suitable condi­tions of preserva­tion. Can we understand all these adapta­tions, without a belief in the heredity of acquired characters? As a matter of fact the tenacity with which some authors cling to such a belief is dictated by the idea that this is the only alternative to the supra-naturalistic or vitalistic ideas. The writer is of the opinion that we do not need to depend upon the assump­tion of the heredity of acquired characters, but that physio­logical chemistry is adequate for this purpose.

The earlier writers explained the growth of the legs in the tadpole of the frog or toad as a case of an adapta­tion to life on land. We know through Gudernatsch that the growth of the legs can be produced at any time even in the youngest tadpole, which is unable to live on the land, by feeding the animal with the thyroid gland. As we have stated in Chapter VII, it is quite possible that in nature the legs of the tadpole begin to grow when enough of the thyroid or a similar compound has been formed or is circulating in the animal.

It might justly be claimed as a case of adapta­tion that the egg attaches itself to the wall of the uterus and calls forth the forma­tion of the decidua. We have mentioned the observa­tion of Leo Loeb that the corpus luteum of the ovary gives off a substance to the blood which alters the tissues in the uterus in such a way that contact with any foreign body (e. g., the egg) induces this decidua forma­tion. Again what appeared as adapta­tion when unknown turns out to be a result of the action of a definite chemical substance circulating in the body.

It appears as a case of adaptation that the eggs of the majority of animals cannot develop without a spermato­zoön, and yet we can imitate the activating effect of a spermato­zoön on the egg by definite chemical compounds, which leads to the sugges­tion that the activating effect of the spermato­zoön on the egg might be due to the fact that it carries such a compound.

The wonderful adaptations exhibited in the mating instincts seem to be due to definite substances secreted by the sex glands, as was shown by Steinach (Chapter VII). Here, again, the process as popularly conceived, is the reverse of the truth; those survive that have the equipment,—they did not acquire the equipment under the influence of environ­ment.

It is absolutely imperative for green plants that their stems and leaves be exposed to the light since only in this way are they able to form carbohydrates; and it is equally essential that the roots should grow into the soil so that the plant may get the nitrates and phosphates required to build up its proteins and nucleins. This result is, in the language of adapta­tionists, brought about by an adaptive response of the plant to the light. In reality this adaptive response is due (Chapter X) to the presence of a photo­sensitive substance present in almost all green plants.

Lewis has shown that if the optic cup is transplanted under the skin of a young larva into any part of the body the skin in contact with the optic cup will form a lens; it looks as if a chemical substance from the optic cup were responsible for the forma­tion of the lens.

These examples might be multiplied indefinitely. They all indicate that apparent morpho­logical and instinctive adapta­tions are merely caused by chemical substances formed in the organism and that there is no reason for postulating the inheritance of acquired characters. We must not forget that there are just as many cases where chemical substances circulating in the body lead to indifferent or harmful results. As an example of the first type, we may mention the existence of helio­tropism in animals living in the dark, of the latter type, the inheritance of deficiencies like colour-blindness or glaucoma.

While it is possible for forms with moderate disharmonies to survive, those with gross disharmonies cannot exist and we are not reminded of their possible existence. As a consequence the cases of apparent adapta­tion prevail in nature.

The following observa­tion may serve to give an idea how small is the number of existing or durable forms compared with the number of forms incapable of existence. We have mentioned the fact observed by Moenkhouse, the writer, and Newman, that it is possible to fertilize the eggs of each marine bony fish with the sperm of practically every other marine bony fish. The number of teleosts at present in existence is about ten thousand. If we accomplish all possible hybridiza­tions, one hundred million different crosses will result. Of these only a small fraction of one per cent. can live (see Chapter I), and it is generally the lack of a proper circula­tion which inhibits them from reaching maturity. It is, therefore, no exaggera­tion to state that the number of species existing today is only an extremely small fraction of those which can and possibly do originate, but which escape our notice and disappear because they cannot live or reproduce. If we consider these facts we realize that the mere laws of chance are adequate to account for the fact of the apparently purposeful adapta­tions; as they are adequate to account for the Mendelian numbers.