OTHER THEORIES OF SPECIES-FORMING. 237 



"2. Keine konstante Varietat oder Art entsteht ohne 

 Ausscheidung einzelner oder weniger Individuen von der 

 Stammart und ohne Ansiedelung an einem neuen Standort, 

 weil Massenkreuzung und Gleichheit der Lebensbedin- 

 gungen in einem zusammenhangenden Wohngebiet immer 

 absorbierend und nivellierend wirken miissen und indivi- 

 duelle Variationen stets wieder in die Stammform zuriick- 

 drangen." 



Wagner's 6 long series of interesting papers and addresses 

 are crammed with facts of plant and animal geography, 

 taxonomy and palaeontology, and with keen interpretation 

 of these facts, and clear and incisive formulations of his few 

 generalisations. 



One of the most ardent present-day upholders of the 

 .species-forming potency of geographical isolation is David 

 Starr Jordan, the foremost American student of the classi- 

 fication and distribution of fishes. From a recent paper 7 

 I abstract the following brief statements of his beliefs con- 

 cerning the character and results of the influence of geo- 

 graphical isolation. 



"It is now," writes Jordan, "nearly forty years since 



Moritz Wagner (1868) first made it clear that geographical 



isolation (raundiche Sonderung] was a factor 



Jordan on . 



geographic or condition in the formation of every species, 

 isolation, mce ^ Qr tribe of animal or plant we know on 



the face of the earth. This conclusion is accepted as almost 

 self-evident by every competent student of species or of 

 the geographical distribution of species. But to those who 

 approach the subject of evolution from some other side the 

 principles set forth by Wagner seem less clear. They have 

 never been confuted, scarcely even attacked, so far as the 

 present writer remembers, but in the literature of evolution 

 of the present day they have been almost universally ignored. 

 Nowadays much of our discussion turns on the question of 

 -whether or not minute favourable variations would enable 



