2 THE QUANTITATIVE METHOD IN BIOLOGY 



supposed mineralogist : we may investigate the transmission 

 of the properties (characteristics) by inheritance, the order 

 in which they appear during individual development and, in 

 many cases at least, the order in which species and groups of 

 species have appeared during the geological ages. We may 

 also modify the properties of a given species and bring about 

 the appearance or disappearance of some of them by modifying 

 the conditions of existence (external causes). In spite of all 

 this, however, our classifications are of an empirical nature. 



§ 4. — At first sight it seems impossible to accept the sugges- 

 tion that a sufficient diversity exists in the chemical com- 

 position of the living substance to explain the existence of 

 millions of animal and vegetable species. This difficulty dis- 

 appears when we look upon the living substance of each species 

 as consisting of a mixture of a rather small number of chemical 

 entities — the components being very probably proteins (or 

 substances related to the proteins) and each specific mixture 

 differing from all others by at least one component. 



If we suppose that the components of all the existing species 

 are thirty in number and that twenty of them occur in each 

 species, we realize the possibility of an enormous number of 

 specific mixtures differing from each other by one, two or 

 more components. ^ If we suppose, moreover, that in certain 

 species twenty, in others nineteen, eighteen, etc., components 

 occur, the number of possible mixtures becomes practically 

 unlimited, just as is the case with the number of living and 

 fossil species. 



§ 5. — In the present state of science it is impossible to verify 

 the above suggestion. It is, however, made admissible by the 

 following facts : — 



(i) A large number of the combinations of carbon may be 

 brought into natural groups, each group consisting of one or 

 several series, each series including several terms. 



EXAMPLE : one of the most important groups consists of 

 the glycerides of the fatty acids (fixed oils and fats). In this 

 group we find numerous series ; for instance, the glycerides of 

 the acids of the 



acetic series {CJlcJd^, 

 acryUc (oleic) series (CnH2n-202), 

 linolic series (CnH 211.402), 

 Unolenic series {Cj^^n.fi^fy ^tc.^ 



1 The number of such specific mixtures differing by one component at least 

 is 30,045,015. 



2 In EDWARD THORPE, A Dictionary oj Applied Chemistry, vol. iii. (1912), 

 p. 744, nine series are mentioned. 



