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.