OTHER THEORIES OF SPECIES-FORMING. 235 



natural selection might modify species and even produce 

 continuous evolution it could never differentiate species, 

 that is, produce new species. It could never, in Wagner's 

 belief, produce the actual condition which we know to exist 

 in the present-day and past (now extinct) animal kingdom, 

 this condition being the existence of hosts of distinct, though 

 related, animal species or kinds. Wagner's travels included 

 journeys to North, Central, and South America, West Asia, 

 and North Africa. His first clear enunciation of his theory, 

 in which pronouncement he took definite stand against 

 the claimed capacity of Darwinian selection to produce new 

 species, was in 1868. in a paper read in Munich, entitled 

 "Die Darwinische Theorie und das Migrationsgesetz der 

 Organismen." From the time of the appearance of this 

 first paper until within a year or two of his death, Wagner 

 steadily wrote and fought for his theory, but without gain- 

 ing for it any such wide or authoritative acceptance as he 

 hoped. In a letter dated August 30, 1884, Wagner pathet- 

 ically writes, "Ich sterbe mlt der '{jberzeugung, dass man 

 dies wenigstens nach meinem Tode anerkennen wird" 



Wagner's theory included not only the characteristics 

 already pointed out as the basis of all theories of the influ- 

 ence of isolation in species-forming, but the assumption that 

 all species of animals have a strong tendency, or are con- 

 stantly attempting, to "spread out"; that is, have a driving 

 instinct of migration and dispersal. The basis of this 

 tendency is undoubtedly the overcrowding in the immature 

 stages and in times of short food-supply or untoward exter- 

 nal conditions of temperature, humidity, etc. This tendency 

 to movement is Wagner's "Migrationsgesetz," and the out- 

 come of it is to bring about conditions of topographic and 

 geographic isolation among all kinds of animals. While in 

 his first papers Wagner looked on his theory as a sort of 

 supporting or auxiliary theory to that of natural selection, 

 he soon began to see in it, calling it now by the name of 



