10 THE NATURE AND ORIGIN OF LIFE 



of the microscope, was the cellular theory, which was thus 

 expressed : 



Every living being is composed of one or several cells. 



This was a general law which concerned the totality of 

 animals and plants : it was therefore a chapter of Biology. 

 We shall see what is to be thought, in strict science, of 

 this unity of structure expressed by the cellular theory ; 

 but, from the start, it gives us an important idea concerning 

 the dimension of the structural phenomena that are possible 

 characters of life. The first structural phenomena which 

 are really common to all living beings, as discovered by means 

 of the microscope in going down the scale of measurable 

 magnitudes, belong to the order of cellular dimensions, that 

 is, they are already far below a millimetre. Without in- 

 sisting here on the exact dimension, the mere discovery that 

 such a dimension exists at once gave the idea that phe- 

 nomena really characteristic of life take place within the 

 neighbourhood of a certain degree of the scale of magnitudes. 

 Anatomy, thus transported by the aid of the microscope into 

 this particular region of measurable things, now took the 

 name of Histology or Cellular Anatomy. Only this part of 

 anatomy can furnish Biology with appreciable results. 



It was easier for physiology to give results of a general 

 order ; questions of nutrition and respiration soon showed 

 the importance of Chemistry in the realization of vital phe- 

 nomena. One of the biological conclusions earliest known 

 was the necessity of oxygen for maintaining life both in 

 animals and plants ; therefore there is chemistry in life, 

 that is, phenomena that take place on a scale of magnitudes 

 below that of cellular dimension, in the order of dimension 

 of molecules and atoms. This second conquest of science 

 helped to assign the place of life more exactly among the 

 other phenomena of nature ; we see from it that biological 



