14 CERTAIN EVIDENCE OF 



That the morphological conception of a Species is 

 not a positive but only a relative conception, and that 

 it has no other absolute or positive value than those 

 other similar system-categories sports, varieties, races, 

 tribes, families, classes is now acknowledged by 

 every systematiser who forms an honest and unpre- 

 judiced judgment of the practical systematic distinction 

 of species. From the very nature of the case there are 

 no limits to arbitrary discretion in this department, 

 and there are no two systematists who are at one in 

 every instance ; this one separating forms as true 

 varieties which that one does not. (Compare on this 

 point "History of Creation," vol. i., p. 273.) The 

 conception of variety or species has a different value 

 in every small or large department of systematic 

 Zoology and Botany. 



But the conception of species has just as little 

 any fixed physiological value. In respect to this 

 we must especially insist that the question of hybrid 

 offspring, the last corner of refuge of all the defenders 

 of the constancy of species, has at present lost all 

 significance as bearing on the conception of species. 

 For we know now, through numerous and reliable ex- 

 periences and experiments, that two different true varie- 

 ties can frequently unite and produce fertile hybrids 

 (as the hare and rabbit, lion and tiger, many different 

 kinds of the carp and trout tribes, of willows, brambles, 



