40 THE QUANTITATIVE METHOD IN BIOLOGY 



In a similar way the principle of segregation ought to be 

 looked upon as being a biological rule/ in spite of possible 

 exceptions. Therefore the conclusion expressed in the first 

 phrase of § 34 is justified. 



§ 36.— MENDELISM {continued). SIMPLE AND COM- 

 POUND PROPERTIES.— A very important result of the 

 Mendelian experiments is the conception that each property is 

 in itself something definite, in a certain sense a unit which has 

 a proper existence. By hybridization it is possible to trans- 

 plant a given property from one species to another, as if the 

 living individuals were the soil, the substratum on which the 

 property grows. 



We may express this conception in a different way by sajdng 

 that a reaction which results in a certain property may be- 

 come possible in a species a in which it never occurred before 

 when the composition of the living substance of a is altered by 

 hybridization. 



From a number of Mendelian experiments it may be con- 

 cluded that the observable properties ought to be brought into 

 two classes : 



(i) Simple properties (so-called elementary characters 2), 

 which it is impossible to decompose into more properties. A 

 simple property might be called a PRIMORDIUM.^ 



(2) Compound properties, which at first sight seem to be 

 simple, but depend on the coexistence of two or more simple 

 properties. In other words, a compound property is a com- 

 bination of primordia. 



This distinction is of the highest importance for the study of 

 many biological problems, especially in descriptive natural 

 science, in which simple and compound properties are con- 

 tinually confounded. 



A simple property is seen to be an entity. A compound 

 property may be decomposed into its components in several 

 ways, especially by Mendelian segregation. (See § 38.) 



EXAMPLES : In numerous cases a given colour of a species 

 is a primordium, depending on the presence of one colouring 

 substance. When the coloured species is crossed with a 

 colourless one, segregation produces in the second hybrid 

 generation (Fg) two sorts of specimens : coloured and colour- 



1 1 avoid purposely the term law. See, on the distinction between law and 

 rule, my lecture on "The Place of Science in History," Mem. and Proceed. 

 Manchester Liter, and Philos. Soc, vol. lix., Part III. (1914-1915), p. 35. 



2 I avoid the term elementary. This term has various significances (see the 

 dictionaries) and is therefore one of the rather numerous linguistic nuisances 

 which embroil our notions and bring confusion to the mind of the biologist. 



^ This term is used by Lucretius in the sense of chemical element, thus in the 

 sense of a simple something. 



