ADAPTATION TO ENVIEONMENT WAKDLAW 397 



tion. We must study the distribution of relatively simple inorganic 

 compounds, rather than that of the highly complex organic sub- 

 stances. Only when comparisons are made on the basis of such 

 broad distinctions of form as that between organisms having a 

 closed circulatory system and those without it are correspondingly 

 fundamental dill'erences of chemical properties to be discerned. 



THE PROPORTIONS OF CERTAIN CONSTITUENTS OF THE ORGANISM 

 AND ITS ENVIRONMENT 



There is no doubt that the unicellular organisms represent an 

 earlier stage in the evolution of living things than do the metazoan 

 or metaphytan forms. They probabl}^ flourished in the primordial 

 oceans for long periods before multicellular organisms made their 

 appearance. They must have been much more closely related to 

 the medium from which they had been produced than the later 

 forms. In particular, their inorganic constituents are likely to have 

 corresponded fairly closely with those of the medium which bathed 

 their cells. The differentiation between the medium and its product 

 had not proceeded as far as it has reached in more complex forms 

 of life. 



It has been pointed out, however, that the inorganic composition 

 of these primitive oceans must have been changing all the time. 

 The effect of regulatory mechanisms in the cell would be to hinder, 

 if not entirely to prevent, the changes from reaching the interior 

 of the cell itself. By the time that living organisms had reached 

 the state of complexity of the primitive multicellular structure, 

 we may imagine them as groups of cells permeated by a solution 

 which showed distinct differences in inorganic composition from 

 that of the solution which bathed them. At first the surrounding 

 medium would have free access to the cells of such a simple organ- 

 ism. But, as the complexity of the organism developed, access 

 would become restricted to certain channels forming a primitive, 

 open, circulatory system. The next stage in complexity would be 

 the closure of these channels against the free ingress and egress of 

 the surrounding medium, and the development of a closed circulatory 

 system. 



At this stage of evolution, the first barrier controlling the ex- 

 changes which take place between the cell and its environment 

 would no longer be situated on the surface of the cell itself. Any 

 changes which reached the liquid actually surrounding the cells 

 would first be subject to the regulatory mechanisms in the outer 

 surface of the organism and in the circulating fluid. The cell 

 would be provided with an immediate environment to some extent 

 under the control of the organism itself. 



