THE BIOLOGY OF THE CELL SURFACE 



If, however, we assume that this combination of com- 

 pounds separated from the environment was first purely 

 homogeneous throughout, then there must soon have come 

 a time when factors in the environment played upon this 

 structure and so modified, if they did not determine, its 

 behavior. It would be difficult to imagine a structure 

 made up of the same elements or compounds found in the 

 environment and maintaining its separateness from the 

 environment without some structural difference from it. 



The moment that such a peculiar combination of com- 

 pounds arose, it assumed life; it had response to its 

 environment, both because it arose from it and had to exist 

 in it.^ 



Living substance can not be considered abstracted either 

 from time or from space. The organism can not be separated 

 from its environment; they form together one inter-acting 

 system. Two predominating characteristics exhibited by 

 living organisms are: first, those changes which are time- 

 ordered; and second, those which are environment-condi- 

 tioned. Thus, the organism — a single cell, for example — 

 changes from moment to moment and the rate of such 

 changes becomes its differentiating characteristic; within 

 the organism these changes tend to run in one direction — 

 the building up of protoplasm from simpler compounds 

 while life lasts. External to it the environment plays a 

 part in yielding up the raw material for these changes 

 and setting the conditions for the reactions in the living 

 matter. In a certain sense we should not speak of the "fit- 



^ Although there is the possibility of life below that size that can 

 be made visible by the microscope, zve must admit that we know little 

 concerning the organization or structure of such ultramicroscopic 

 organisms. Therefore, zve might most profitably begin with a 

 hypothetical organism whose size is within the range of resolution 

 by the microscope. Nevertheless, what is said of such a hypothetical 

 structure might also hold for one of ultramicroscopic size. 



