HISTORY OF ZOOLOGY. 33 



pletely in the lurch when it is called upon to show the dis- 

 tinctions between what should be called a species and what a 

 variety. Therefore there remains open to the systematist 

 only one resource for attaining his aim, i.e., by summoning 

 Physiology to his aid. This has been done, and it has 

 disclosed considerable distinctions in reproduction. We 

 should expect a priori that the individuals of different 

 species would not reproduce with each other; on the other 

 hand under normal conditions the individuals of one and 

 the same species, even though they are of different varieties 

 or races, should be entirely fertile. One must beware of 

 arguing in a circle in proof of these two propositions ; it 

 would be an argument in a circle if an experimenter 

 should regard t\yo animals as representatives of one 

 species only because they proved to be fertile together, 

 \vhile under their former relations he assigned them to dif- 

 ferent species. Rather the question for him must read : 

 does physiological experiment lead to the same systematic 

 distinctions as does the common systematic experience, 

 viz., the depreciation of constancy and the divergence of 

 distinguishing characters ? 



B. The Intercrossing of Species. The region which we 

 here enter is as yet far from being sufficiently worked through 

 experimentally ; yet already some general propositions can 

 be set up: (i) that not a few so-called " good species" can 

 be crossed with one another; (2) that in general the diffi- 

 culty of crossing increases, the more distant the sys- 

 tematic relationship of the species used ; (3) but that 

 these difficulties are by no means directly proportional to 

 the systematic divergence of the species. The most 

 favorable material for research is furnished by those ani- 

 mals in the case of which artificial fertilization can be car- 

 ried out, i.e., of which one can take the eggs and sperma- 

 tozoa and mix them independently of the will of the 

 animals. Thus hybrids have been obtained from species 

 which belong to quite different genera, while very often 

 nearly-related species will not cross. Among fishes we 

 know hybrids of Abrainis brania and Blicca Bjorkna, of 



