RESPIRATION 389 



and their parts, and investigate their mechanical and chemical 

 properties. This enables us to predict certain points in their be- 

 havior, as shown, for instance, in Chapters IV and V. But when we 

 look more closely it becomes quite evident that the knowledge 

 we gain from mere physical and chemical examination hardly 

 touches any fundamental physiological problem. We cannot es- 

 cape from the relativity of the phenomena we are dealing with. 



The only way of real advance in biology lies in taking as our 

 starting point, not the separated parts of an organism and its 

 environment, but the whole organism in its actual relation to 

 environment, and defining the parts and activities in this whole in 

 terms implying their existing relationships to the other parts and 

 activities. We can do this in virtue of the fundamental fact, which 

 is the foundation of biological science, that the structural details, 

 activities, and environment of organisms tend to be maintained. 

 This maintenance is perfectly evident amid all the vicissitudes of 

 a living organism and the constant apparent exchange of material 

 between organism and environment. It is as if an organism al- 

 ways remembered its proper structure and activities ; and in repro- 

 duction organic "memory," as Hering figuratively called it, 2 is 

 transmitted from generation to generation in a manner for which 

 facts hitherto observed in the inorganic world seem to present no 

 analogy. We can discover and define more and more clearly by 

 investigation these abiding details of structure and activity, dis- 

 tinguishing accidental appearances from what is really main- 

 tained; and this process of progressive definition is the work of 

 the biological sciences. 



If we look back on the general outcome of the investigations 

 summarized in this book, it is evident that the progress made has 

 consisted in distinguishing underlying identity of activity amid 

 superficial appearances which at first sight present confusion. In 

 the second and third chapters it was shown that behind the ir- 

 regularities of ordinary breathing the mean pressure of CO 2 in 

 the alveolar air is maintained steady within narrow limits for 

 each individual ; and in a later chapter it was more definitely 

 shown that this implies a similar steadiness in the CO 2 pressure of 

 the respiratory center. In Chapter VIII this conclusion is widened 

 by the evidence that CO 2 pressure is only important as an index 

 of blood reaction, and that it is blood reaction, and not mere pres- 

 sure of CO 2 , that is kept so constant by the breathing. In Chap- 



2 E. Hering, Memory as a Generalized, Function of Organized Matter (1870). 

 English Translation, Chicago, 1913. 



