PART IX 



APPLICATION OF THE QUANTITATIVE METHOD— 

 THE DESCRIPTION OF SPECIES 



§ 143.— VARIOUS APPLICATIONS OF THE QUAN- 

 TITATIVE METHOD —In this book I have pointed to a 

 number of possible appHcations of the quantitative method — 

 for instance : 



(i) The construction of embryological curves, based upon 

 the measurement of a sufficient number of primordia in succes- 

 sive states of development. 



(2) The investigation of the structure of animals and plants 

 by means of gradation curves based upon the measurement of 

 primordia along the axes. 



(3) The quantitative investigation of the influence of external 

 conditions of existence (plasticity). 



(4) The quantitative description of the results of experiments. 



(5) The quantitative description of species. 



(6) And, in general, the determination of constants, by which 

 it becomes possible to replace terms by figures in any descrip- 

 tion whatever and to obtain exact expression of observed facts. 



In my paper on Mnium (mentioned, p. 69) I have applied the 

 method to the description of species and also to the description 

 (bertillonage) and the identification of specimens, replacing the 

 classic dichotomous tables by tables of constants. I have 

 taken as example ten British species of the genus Mnium. I 

 have applied the same method on the species of the genus 

 Carabus and on the GRAMINEiE of the Manchester district. ^ 

 From the fact that the adopted method gives satisfactory 

 results in each of those three widely different groups of living 

 beings, one may conclude that it is applicable to all animals 

 and plants. In certain cases, however, measurements are 

 rendered difficult by practical obstacles — for instance, by the 

 contractility of certain animals {Polypes, Bryozoa, Molluscs, 

 etc.), by the astonishing plasticity of certain plants (many lower 

 Fungi, etc.), a.s.o. It may be hoped that such technical diffi- 

 culties will be surmounted one after another. 



§ 144.— BREACH OF CONTINUITY IN THE DEVELOP- 

 MENT OF BIOLOGICAL SCIENCE.— I wish to emphasize 



1 The total number of measurements which I have hitherto carried out in 

 the three mentioned groups may be estimated at more than 300,000. 



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