592 Species According to the Theory of Mutation. 



tlie l)est varieties are, as a rule, not united by transitional 

 forms with the parent species ; in the second place, trans- 

 gressive varialjility tends to obscure boundaries where 

 they really exist. These limits are often overlooked in 

 the descriptive method, and the search for them can only 

 be carried out on experimental and statistical lines. With 

 good right De Candolle speaks in such cases of provi- 

 sional species.-^ 



§ 5. THE PARALLEL BETWEEN SYSTEMATIC AND 

 SEXUAL RELATIONSHIP. 



Since the doctrine of descent now commands general 

 recognition, it is desirable that the systematic divisions 

 should be an expression of the various degrees of rela- 

 tionship. Even before the appearance of Darwin's 

 works it was recognized that the task of systematic biol- 

 og\' as a descriptive and classificatory science was differ- 

 ent from the mere question of actual relationship. To in- 

 vestigate this and, where possible, to bring the divisions 

 of the natural system into harmony with it, these were 

 the ends which the pioneers in the study of hybridization 

 had continually in view. 



The result did not, however, correspond to tliis ex- 

 pectation. We have not, as yet, succeeded in bringing 

 into harmony the study of hybridization with that of 

 systematic biology. Nageli expressed this incompati- 

 bility most clearly by introducing his conception of sex- 

 ual affinity. The degree of this affinity between two 

 types was judged first by the degree of their fertility 

 when crossed with one another, and, then, by that of the 

 fertility of the hybrids thus produced. 



* Alph. de Caxdoi.le, La PhyfogropJiic, pp. 98, 167. 



