out how much CaCl2, was required to allow them to swim permanently at the surface.

In order to interpret these figures correctly we must remember that we are dealing with two different antagonisms, one between the salts with univalent and bivalent metals and the other between Mg and Ca. The former antagonism is satisfied by the addition of Mg, inasmuch as enough Mg was present for this purpose in all solutions. What was lacking was the balance between Mg and Ca. The experiments in Table XIX therefore answer the question of the ratio between Mg and Ca. If we consider only the concentrations of Mg between 2.5 and 10.0 c. c. 3% m MgCl, -which are those closest to the normal concentration of Mg in the sea water-we notice that CCa must vary in proportion to CMg. If we now combine the results of this and the previous paragraph we may

express them in the form of the theory of physiologically balanced salt solutions, by which we mean that in the ocean (and in the blood or lymph) the salts exist in such ratio that they mutually antagonize the injurious action which one or several of them would have if they were alone in solution. This law of physiologically balanced solutions seems to be the general expression of the effect of changes in the constitution of the salt solutions for marine or all aquatic organisms.

This chapter would not be complete without an intimation of the rôle of buffers in the sea water and the blood, by which the reaction of these media is prevented from changing in a way injurious to the organism. These buffers are the carbonates and phosphates. Instead of saying that the organisms are adapted to the medium, L. Henderson has pointed out the fitness of the environment for the development of organisms and one of these elements of fitness are the buffers against alterations of the hydrogen ion concentration. The ratio in which the salts of the different metals exist in the sea water is another. It is obvious that the quantitative laws prevailing in the effect of environment upon organisms leave no more room for the interference of a "directing force" of the vitalist than do the laws of the motion of the solar system.

This theory was first expressed by the writer in Am. Jour. Physiol., 1900, iii., 434.

Henderson, L., The Fitness of the Environment. See also Michaelis, L., Die Wasserstoffionenconzentration. Berlin, 1914.

CHAPTER XII

ADAPTATION TO ENVIRONMENT

I. 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,' 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

Loeb, J., Der Heliotropismus der Tiere und seine Übereinstimmung mit dem Heliotropismus der Pflanzen. Würzburg, 1890 (appeared in 1889).

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