398 ANNUAL REPORT SMITHSONL\N INSTITUTION, 19 31 



During all the period necessary for these developments, the change 

 of composition of the surrounding medium would be still in prog- 

 ress. But once the action of a closed circulatory system became 

 elective, the changes in composition of the medium bathing the 

 cells would not proceed so rapidly. Although the composition of 

 the external medium might continue to change, the changes would 

 only reach the circulating fluid, the internal environment of the 

 cells, in a modified form. The organism would tend to preserve the 

 composition of the fluid with which it had been bathed while a 

 closed circulatory system was being evolved. 



Such an organism would be capable of very considerable inde- 

 pendence of its external medium, an independence which would 

 increase as its external regulatory mechanisms grew in perfection. 

 Not until living organisms had reached this stage of complexity 

 would they become able to emerge from the medium in which they 

 had evolved, and their appearance as land forms of multicellular 

 organisms become possible. 



It is well known that the inorganic composition of the cells of 

 various tissues of an organism differs materially from the inorganic 

 composition of the circulatory fluid. According to the hypothesis 

 outlined above, the inorganic constituents of the cells ought to cor- 

 respond to those of sea water at an early stage in the evolution of 

 living things. The composition of the circulating fluid ought, on 

 the other hand, to correspond, with regard to its inorganic constitu- 

 ents, to the composition of sea water of a much later period. 



Macallum (1926) has collected a good deal of evidence in de- 

 veloping this hypothesis. The most striking difference between the 

 inorganic constituents present in the cells and those in the circulating 

 fluids of one of the higher animals is the relative abundance of salts 

 of potassium in the former and of salts of sodium in the latter. 

 There is a remarkably close correspondence between the proportions 

 of these two elements in the circulating fluid of the most highly 

 organized living forms which have so far been developed and in the 

 sea water of the present day. If the comparison could be made 

 with the composition of the sea water during the period in which 

 the vertebrates first appeared, the correspondence would perhaps 

 be still closer, owing to the slightly greater pro^Dortion of potassium 

 in the sea water of that period. 



The difficulties in the way of forming an accurate estimate of the 

 composition of sea water during the comj^aratively recent geological 

 epoch in which vertebrates appeared are great enough. Much greater 

 are the difficulties in the way of an attempt to form a similar 

 estimate of the composition of the sea water of the remote age dur- 

 ing which unicellular organisms were evolving. Even though no 



