82 College of Forestry 
But what impresses one especially is the fact that for 
temperate and boreal Europe, Asia and North America, the 
plant life appears so generally to have been derived from a 
common source. ‘The floristic stock, so to speak, is largely 
the same. The growth forms (as referred to earlier, p. 23) 
are largely the same and the vegetation aspects likewise. So 
frequently, one finds the same species in all three continents 
(see the notable case of the arctic indicator species of Mt. 
Marey, all of which fall in this category, though there are 
others which have not been found in all three continents) or 
at least closely related species (as our paper birch which 
Fernald * calls a variety of the European white birch). This 
agreement among the species of the three continents was 
naturally emphasized first as to the higher plants — trees, 
flowering plants, angiosperms and gymnosperms, generally — 
as to certain pteridophytes also and these more as the group 
became more generally studied. More recently this identity 
or close similarity of species has also been noted for the lower 
groups and more and more as each group (for example of 
fungi) is closely revised by special students of them, the 
conviction comes that the relationship is very thorough-going 
indeed; that not merely species, but especially the composi- 
tion and aspect of vegetation types (plant societies) points, 
as one may say, to a common developmental experience. Thus 
one notes the agreement as between swamp plant societies, 
bog societies, the vegetation of sand plains in Europe and 
America (where these cases have been especially studied) 
and a fundamental similarity as between the diverse mem- 
bers of the high forest or climax forest of the three con- 
tinents in similar zones, from dominant trees down to forest 
floor herbs, mosses, liverworts, lichens and fungi. Close com- 
parison studies of these matters would seem to be the next 
inviting field made possible especially by the rapid progress 
in monographing the various taxonomic groups. 
The era of development to which we must look as offering 
an explanation of these fundamental agreements between 
1 Fernald, M. L. The relationships of some American and Old-World 
birches. Amer. Jour. Sci., 14:1902, pp. 167-194. 
