12 THE BIOLOGICAL PROBLEM 



methods which mutually check each other. The experimental 

 facts have been studied quantitatively, and our conclusions 

 are often derived from their mathematical relation. We do 

 not start from one or more postulates, but from measurements 

 and velocities. These researches were not undertaken with 

 the object of arriving at a definition of time, but on the 

 contrary in order to elucidate certain very definite biological 

 problems. The conclusions imposed themselves on us when 

 the experimental work was ended. That is why it is necessary 

 to give the reader a general idea of the biological problem and 

 of the methods by which it can be examined in detail, so that 

 he may follow progressively the path which led us to these 

 conclusions. It is important for him to be fully convinced 

 of the part played by the physico-chemical and chemical 

 mechanisms in life phenomena in order to understand, for 

 example, the value of the argument derived from the demon- 

 stration of the activity of the temperature coefficient (Van't 

 Hoff constant) in the appreciation of time. It is essential 

 that he should understand the role of the chemical reactions 

 in the organisms. It is indispensable for him to know in 

 detail the mechanism of the cicatrization of wounds and of 

 tissue-culture, so that he can trust the calculations on which 

 the whole work is based, and realize the importance of the 

 quantitative checks on which our reasoning rests. The 

 elementary notions to which we constantly refer will some- 

 times be briefly dealt with, so as to avoid the necessity for the 

 reader to consult technical books or papers in search of mere 

 definitions. 



In other words, we have tried to answer beforehand the 

 principal questions which might be put to us and the objections 

 which might be raised. We are not under the illusion that 

 we have completely succeeded, but we have done our best. 



Our plan is the following: 



I . We will show that the modern conception of a living 

 organism enables us to study it from different points of view 

 which can be roughly classed in three groups, each of which 

 imposes a distinct category of methods: 



