MODERN BIOLOGY 613 



X. The Idea of Species and Some Problems in Connexion Therewith 



In a popular account of the results of heredity research published some years 

 ago occurs the sentence: "For the very reason of the great number of fresh 

 facts that modern heredity-research has brought to light, chaos prevails at 

 present in regard to the views on the formation of species." These words 

 characterize both the situation as regards the problem of species at the pres- 

 ent day and the causes that have brought it about. Modern heredity-research 

 has completely dislocated the circles drawn by the old morphological classi- 

 fication. Linn^eus's idea of species, it will be remembered, was essentially 

 genetical- he counted as many species as had been created in the begnining, 

 or in later years, at any rate some species created in the dawn of time in 

 respect of each genus, out of which the other species have since been evolved. 

 This idea of species could very easily be reconciled with the idealistic idea 

 of species which has existed since the days of Greek philosophy and which 

 the biology of the romantic period preferred; in those days greater attention 

 was paid to the idea expressed in the species form than to the question of 

 origin Darwinism brought the genetical idea of species once more into re- 

 pute To discover the origin of the different forms of life by a close compari- 

 son of their external and internal structure was, according to Gegenbaur, 

 Haeckel, and their disciples, the end of biology- thus, a natural classification 

 system was to be created with species based upon true relationship — species 

 which it is true, must be assumed to vary and overlap, but which in their 

 typical forms could be determined and characterized. Nevertheless, this ge- 

 netical idea of species rested upon an indispensable proviso — namely, that 

 from resemblance one could positively conclude affinity: the greater and the 

 more universal the resemblance, the closer the affinity. Every species, and 

 even every variety, had a common origin, as proved by the mutual resem- 

 blance between its individuals. It is this foundation for the idea of species 

 that modern heredity-research has undermined; it has clearly demonstrated 

 that very close morphological resemblance can in certain cases be due to 

 entirely different causes. It is not outward resemblance, but the concurrence 

 of hereditary factors that proves true affinity; that is to say, it is not phe- 

 notypical, but genotypical resemblance that determines the affinity. But 

 nowadays in all systematical works the species are described entirely ac- 

 cording to phenotypes; genotypical agreement can be ascertained only by 

 experimental means. And in practice, of course, this can take place only on 

 a small scale; the plant-geographer who makes records of localities and dis- 

 tribution charts at home or abroad would be stranded if, every time he sees 

 a form he were compelled to carry out hybridizing experiments with it in 

 order to establish its identity, and this applies also to the morphologist and 

 the systematist. In these circumstances it seems to be absolutely necessary 



