THE QUANTITATIVE METHOD 

 IN BIOLOGY 



PART I 



THE NOTION OF SPECIES IS A CHEMICAL NOTION 



§ 1. — Each animal and vegetable species ^ differs strictly 

 from all others by the chemical composition of its living 

 substance. 



§ 2. — The so-called characteristics of each species are the 

 product of reactions, in which there intervene on the one hand 

 the external causes which affect the individuals during their 

 development and on the other hand the living substance of the 

 species under consideration. 



§ 3. — Our knowledge of the chemical composition of living 

 substance is too incomplete to found a RATIONAL classifica- 

 tion of plants and animals based upon differences of chemical 

 constitution. In the present state of science our classifications 

 are founded merely on differences in observable characteristics 

 (properties). 



In our study of living beings we find ourselves in much the 

 same position as a mineralogist would be if he were ignorant of 

 the chemical constitution of the minerals, and were thus limited 

 to the study of their properties, such as crystalline form, optical 

 properties, density, hardness, colour, etc., and were to try to 

 found on those data a classification of the mineral species.'^ 

 Fortunately, we dispose of a larger number of facts than our 



^ The term species is used here in its most general significance (specific form) 

 — that is to say, in the sense of a form of life (species properly so called, elemen- 

 tary species, race, etc.) characterized by hereditary properties. 



2 See Sir HENRY ALEXANDER MIERS, F.R.S., The Old and the New 

 Mineralogy, in Trans. Chemical Society, 1918, vol. cxviii., pp. 364-386. In this 

 interesting paper the author has recorded that, in former times, the characters 

 of minerals which were used for the purpose of discriminating between species 

 and of classifying them were the external or natural history properties (specific 

 gravity, colour, crystalline form, etc.). Later on (DANA, 1850) a new classifi- 

 cation of the minerals, based upon their chemical composition, was adopted. 



