270 THE PROPERTIES OF PROTOPLASM 



approach toward the vertebrate composition of the serum is already 

 indicated, as the following figures reveal: 



Fluid. Na K Ca Mg 



Ocean-water 100 3.6 3.9 12.1 



Serum of Limulus polyphemus .100 5.6 4.1 11.2 



Serum of Homarus americanus .100 3.7 4.9 1.7 



According to Macallum the development of a kidney in the proto- 

 vertebrate forms from which vertebrates have arisen, fixed the com- 

 position of the tissue-fluids of the vertebrata for all time, since the 

 primitive kidney was adapted to the concentration and proportions of 

 the mineral constituents of the ocean of that period. In the early 

 Cambrian or pre-Cambrian period at which the ancestral forms of the 

 vertebrates arose, the sea-water must have been very much more dilute 

 than it is at present day, because sodium chloride is constantly accumu- 

 lating, since it is not deposited in important amounts in the marine 

 geological formations. Calcium and potassium are deposited from sea- 

 water in the form of limestone and minerals such as glauconite at about 

 the same rate as that at which they are carried into the sea by rivers. 

 Magnesium, however, is increasing in the sea-water not only absolutely 

 but also relatively to the sodium, the rate of deposition being much 

 slower than the rate of addition. It is quite probable, therefore, that 

 the sea-water of the early Cambrian epoch was not only much more 

 dilute than the sea-water of our day, but also contained both absolutely 

 and relatively much less magnesium. 



The blood-serum of mammals therefore resembles a diluted sea- 

 water with the exception that its magnesium content is both absolutely 

 and relatively much lower than the magnesium content of the sea-water 

 of our own day. Just as the homoiothermal animals have acquired a 

 large measure of independence of the temperature of their environment, 

 so, and at an earlier stage of evolution, the vertebrates have acquired 

 a large measure of independence of their osmotic environment, they 

 are "homoiosmotic," while the more elementary forms are "poikilos- 

 motic" and the cells of which they are composed are exposed to all the 

 disadvantages of an irregularly fluctuating milieu. At a still earlier 

 stage of evolution the multicellular organisms acquired, as we shall see, 

 more or less efficient means of maintaining constancy of the reaction 

 or hydrogen ion concentration of their tissue-fluids. Each of these 

 successive stages marked an additional degree of emancipation from 

 the fortuitous inequalities of an unstable environment and a step 

 toward the self-creation of an equable "internal environment/' suit- 

 able for the maximum furtherance of vital activities. 



The mechanism by which this environmental stability is brought 

 about is similar in each of the three cases and consists in a balance 

 between income and output so adjusted that the dissipating agencies 

 (excretory activity of the kidneys, radiation of heat from the surface 

 of the body, and release of carbon dioxide from the lungs, respectively) 



