210 



ancestors who lived in a solution fairly rich in this salt. Macallum (1903, p. 234), 

 struck by the similarity between the proportion of potassium and calcium to 

 sodium in the blood plasma of vertebrates and that in sea water, was led, 

 independently, to advocate the same view. 



The ocean, ever since the first condensation of water on the earth's surface, 

 has been continually receiving salts by dissolving them from its bed and from 

 the contents of the rivers flowing into it. Since the salts are left behind on 

 evaporation, while water vapour is continually rising to form new rivers, which 

 wash away more constituents of the land, it is easy to understand why the 

 total concentration of salts in sea water is, at the present time, so much higher 

 than that which it was at the time when the ancestors of the land vertebrates 

 left it. 



It is generally believed that life began in the ocean and continued in it 

 alone until the close of the Cambrian period. When vertebrates with a closed 

 circulatory system took to the land, they took with them a blood of the same 

 composition, as regards salt, as the sea water which they left behind. 



The Cambrian period was an extremely long one, judging by the thickness of the deposits, 

 amounting to 40,000 feet in British Columbia, and 12,000 feet in Wales, although it varies in 

 different places. It is to be expected, therefore, that the protoplasm would have become 

 adjusted to the salts of the sea during this long period, and that mechanisms would have been 

 produced to maintain the concentration in the blood at the same value. These mechanisms 

 still continue to act since life on land began. 



If this view is correct, the salt composition of the blood represents that of 

 the ocean in the early Cambrian period. As regards the proportion of calcium 

 and potassium in sea water, Macallum points out that, at the present time, the 

 concentrations of these two salts is scarcely changing at all. Calcium is being 

 continually removed by living animals for the formation of bones, coral, shells, 

 etc., as fast as it is supplied by the rivers. Potassium, since the great develop- 

 ment on land of plant life, with its comparatively large content in this element, 

 is supplied by the rivers in much less quantity than it was in early geologic 

 times. The chief difficulty is the magnesium, 1 which is present now in so much 

 greater ratio to sodium in sea water than it is in blood plasma. According to 

 Macallum, the reason is that the magnesium content of sea water is still 

 slowly increasing, so that " in the pre-Cambrian oceans it must have been very 

 small, not perhaps as low as it is in blood plasma, for in the latter the 

 magnesium would only represent the proportion of an earlier period than that 

 in which the circulation became closed, as the tissues would only reproduce 

 the proportion which had by long accommodation become fixed in them. Even 

 the organisms which live in the sea to-day, whose ancestral forms have lived 

 in the sea since the Cambrian, do not take up the magnesium from the sea 

 water in the full proportion which it has in the latter" (1904, p. 8 of 

 reprint). Chemical changes by which magnesium chloride in the primeval 

 ocean became precipitated as magnesia must also be taken into account 

 (1904, p. 12). 



A further interesting question concerns the salts of the cells themselves, 

 a more difficult problem; but, as Macallum puts it (1904, p. 9 of i-eprint), 

 " If the blood plasma of vertebrates, because of the forces of heredity, 

 reproduce the proportions which obtained in pre-Cambrian oceans, why should 

 not the cells of the tissues, because of the same forces, reproduce in them- 

 selves the proportions which obtained in sea water of a much earlier geological 

 period ? " 



There are different questions involved in the discussion of this problem, the consideration 

 of which would lead us too far. The reader interested may refer to the paper quoted, and 

 to a further one on the salts of the blood (1910). 



Whatever may be the final decision on the question, the fact remains that 

 sea water, diluted to the same osmotic pressure as the blood, is a very effective 

 physiological solution, although the amount of magnesium is unnecessarily 

 great. 



Returning to the preparation of such solutions for experimental use, it is found 



