26 THE QUANTITATIVE METHOD IN BIOLOGY 



successive generations. It is more accurate to say that 

 THE POSSIBILITIES ARE HEREDITARY.^ 



To discover all the possibilities of a species, we ought to 

 observe it under all possible conditions of existence. 



A classification of the living species based upon their 

 observable properties is of an empirical nature. A rational 

 classification based upon differences of chemical constitution is 

 impossible in the present state of science. Our classifications 

 are founded on the postulate that specimens in which the 

 same possibilities are hereditary are identical (or similar) with 

 regard to the constitution of their living substance and belong 

 therefore to the same specific form or systematic group (pure 

 line ? ; subspecies, species, etc.). 



We are still ignorant of the mechanical relations which exist 

 between a given property of a species and its chemical constitu- 

 tion. In spite of this, certain hypotheses about the chemical 

 constitution of the living substance (hereditary factors, etc.) 

 may have a heuristic value and render good services. 



§ 30.— QUANTITATIVE DESCRIPTION OF THE PRO- 

 PERTIES. — The properties (characteristics) of animals and 

 plants are ordinarily described by means of terms. This 

 method is unsatisfactory ; terms are vague, and the descrip- 

 tions in faunas and floras, leave us often in doubt as to the 

 identification of a given specimen. Is it possible to give a 

 quantitative description of the living individuals and species, 

 expressing the individual and specific properties by measure- 

 ment ? 



The quantitative method has been already used along five 

 different lines — viz. 



(i) In the description and classification of species. (LIN- 

 NAEUS, LATREILLE, etc. See § 31.) 



(2) In biometrv properly so called. (QUETELET, GALTON, 

 PEARSON, WELDON, etc. See § 32.) 



(3) In the study of hybridization. (MENDEL. See § 33.) 



(4) In phyllotaxis, including floral diagrams. (See § 40.) 



(5) In the measurement of certain physiological functions 

 and corresponding properties. (See § 41.) 



^ In a similar way the state of equilibrium of a chemical entity (species ) 

 which is called its crystalline form depends on a specific possibility which takes 

 the form of an observable property under certain conditions. In chemistry 

 and mineralogy a confusion between the terms possibility and property is of 

 no consequence, any ambiguity being precluded by our knowledge of the 

 chemical structure of the entities. In a living species properties appear and 

 disappear and are altered without any alteration of the possibilities, but the 

 chemical constitution on which the latter depend is up to the present time 

 unknown. Therefore, if we want to avoid ambiguity, we must always bear 

 in mind that the state of equilibrium (see § 43 ) which we call a property is 

 the temporary realization of a possibility (potentiality) of the species. 



