QUANTITATIVE METHOD AND PRIMORDIA 47 



possible along four different lines. Mendelian segregation 

 corresponds to one of these lines : it is a particular example of 

 a phenome7ion the importance of which extends far beyond the 

 limits of Mendelism. It may be anticipated that the study of 

 gradation, plasticity and ontogeny with regard to segregation 

 may throw new Hght upon the facts observed in hybridization, 

 and bring more accuracy to descriptive natural history. 



§ 39.— MENDELISM {continued). THE MENDELIAN 

 METHOD OF DESCRIBING HYBRIDS IS APPLICABLE 

 TO THE DESCRIPTION OF SPECIES.— MENDEL has 

 initiated a new method for the description of hybrids. Accord- 

 ing to this method each hybrid is characterized by the co- 

 existence of two or several properties (primordia). 



EXAMPLES : When two species which differ in two pairs 

 of properties Z>, R and d, r are crossed the F2 generation con- 

 sists of four sorts of specimens characterized by the combina- 

 tions Dd, Dr, Rd, Rr. In a similar way, when the crossed 

 species differ in three pairs of properties D, R; d, r and S, p, 

 eight sorts of specimens are observed in the Fg generation — viz. 



DdB Ddp RdS Rrp 



This method is applicable to the description of all living 

 beings. It may be adopted for the description of subspecies, 

 species, genera, etc., as often as it is possible to discern a suffi- 

 cient number of primordia susceptible of a strict definition. 

 (A general application of the method is possible when the 

 primordia are expressed by measurement. Each description 

 becomes then a combination of figures. See Part IX.) 



It happens very rarely that one property (eigenschap, Eigen- 

 schaft) is a character — that is to say, a distinctive mark {ken- 

 merk, Kennzeichen) — by which a species may be distinguished 

 from others. A species (just as a hybrid) is almost always 

 characterized by the coexistence of several properties : their 

 combination is a character. 



Let us consider the six following properties of a flower : — 

 5 sepals; 5 petals; 5 stamens; i ovary; 5 styles; petals 

 blue. Each of these properties exists in numerous species and 

 is not a characteristic as long as it is taken separately. But 

 flowers in which the six properties exist at the same time are 

 found only in a very small number of species of the genus 

 Linum. If each property was represented by a letter, the six 

 letters being associated into a combination, this would enable 

 us to recognise the two or three species of Linum under con- 



