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. 3⁄8 m MgCl2—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.[273] 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.[274] 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.

CHAPTER XII

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.