60 INTRODUCTION TO GENERAL PHYSIOLOGY 



as we saw, are active in the way of aggregating or precipitating 

 colloids, but there are probably other properties to be taken into 

 account. Potassium appears to be of importance on account of its 

 radio-activity. 



There is an interesting and suggestive fact about the salts 

 necessary in a solution to take the place of blood. Suppose that 

 we take sea water and dilute it so that its osmotic pressure is the 

 same as that of blood. We find that it serves excellently as an 

 artificial fluid, so far as the salts are concerned. Examining it 

 more closely, we notice that the proportion of sodium, potassium, 

 and calcium salts is practically the same as that found to be the 

 best in a mixture made for the purpose. Is this merely accidental ? 

 The blood, as we find, contains the salts of the ocean such as they 

 would be if sea water were less concentrated than it is now. But 

 we know that it has been, through geological ages, continually 

 increasing in salt content, because rivers are always adding salts 

 dissolved from the land by rain, whereas it is only water that 

 evaporates from the ocean. At some period, then, its composition 

 was similar to that of the present land vertebrates as regards 

 inorganic salts. When the ancestral vertebrates, which were formed 

 in the ocean, left it for the land, there is every reason to suppose 

 that the salt content of their blood would be the same as that of 

 the ocean, and that their cell mechanisms would have been adjusted 

 in relation to it. Hence it remained at this point. The geologists 

 tell us that this taking to life on land occurred about the end of the 

 Cambrian period. This period was one of great length, judging 

 by the thickness of the rocks ; so that ample time had passed for 

 the adjustment of the cell mechanisms to the composition of the 

 ocean. We may take it that the blood represents the salt content 

 of the ocean at the end of the Cambrian period. There is, however, 

 one point which requires some further explanation, namely, the 

 high content of the sea at the present time in magnesium salts, 

 which is out of proportion to the other constituents, as compared 

 with blood. There are reasons for believing that magnesium has 

 increased more than the other salts, but further discussion would 

 lead us too far (p., p. 210). 



A further conclusion is suggested. Perhaps the salt content of 

 the cells, which is not identical with that of the blood, may 

 represent the composition of the ocean at a still earlier period. 

 But there are difficult questions involved here. 



The variety of salts required for growth, at all events in the 

 case of plants, and as far as we know in that of animals also, is 

 greater than this. The ordinary mould, Aspergillus, requires for 

 its most rapid growth magnesium, potassium, zinc, and iron as 

 cations ; phosphate, sulphate, and silicate as anions. A sea weed 



