512 RELATION TO ENVIRONMENT. 



both in Europe, North America, and Japan. Evidence of this 

 is shown in the close relationship of the flora of northern 

 Europe, North America, and Japan. Many species and genera 

 of plants are found in these countries which are the same. Under 

 the present conditions of the climatology of the earth, it would 

 be impossible for the plants to communicate to such an extent 

 as to explain the presence in these different continents of such a 

 large number of the same species. While in the seed plants 

 there are many similarities in the flora, and many species and 

 genera are identical ; in the lower forms, among the algae, fungi, 

 liverworts, and mosses, there is an even greater similarity. This 

 leads us to believe that even microscopic plants like the fungi 

 and algae migrated under these conditions along with the seed 

 plants. The parasitic fungi moved along with their hosts, and 

 saprophytic fungi, like the mushrooms, followed the movements 

 of forest trees, growing on dying or dead trunks, upon the leaves, 

 and leaf -mold in the forest. The aquatic fungi and the fresh-water 

 algae likewise moved southward with the aquatic flowering plants. 

 The fact that so many of the fresh-water forms of the fungi and 

 a'gae are identical with many of those in northern Europe sug- 

 gests that in former times the continents in the arctic circle were 

 very near together, if not actually connected, that the climate 

 was milder, and that there was a migration of these fresh-water 

 plants between the continents This might be brought about 

 by a possible continuity of land and fresh-water areas, or through 

 the migration of water-fowl tne spores of algae and fungi cling- 

 ing to their feet could be transported across land areas or chan- 

 nels of salt water, when these were not too wide, and lodged in the 

 fresh-water pools, or lakes, or streams of another near-by conti- 

 nent. 



982. Present climatic pressures. Other climatic pressures 

 also existed and continue to the present time. In the humid 

 tropics large numbers of individuals of different species were 

 propagated, which produced a pressure northward and south- 

 ward from this point, but those moving southward on the north- 

 ern hemisphere come in contact with those moving northward, 



