- 
American Association at Dubuque. 298 
this subject somewhat fully, and tabulated the facts within my 
reach.* 
This was before Heer had developed the rich fossil botany 
of the arctic zone, before the immense antiquity of existing 
species of plants was recognised, and before the publication 
arwin's now famous volume on the Origin of Species 
had introduced and familiarized the scientific world with those 
how current ideas respecting the history and vicissitudes of 
species, with which I attempted to deal in a tentative and 
feeble way. 
western sides of the continents—the one extreme, the other 
mean—was doubtless even then established, so that the same 
Species and the same sorts of species would be likely to secure 
and retain foothold in the similar climates of Japan and the 
Atlantic United States, but not in intermediate regions of dif- 
ferent distribution of heat and moisture; so that different 
Species of the same genus, as in Yorreya, or different genera of 
le same group, as Redwood, Taxodium, and Glyptostrebus, or 
different associations of forest trees, might establish themselves 
Presup an ancestry in pliocene or still earlier times occu- 
Pying the high northern regio nd it was thought that 
occurrence of peculiarly North American genera In Kuro 
* Mem. Amer. Acad., vol. vi. 
