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]