FROM THE STANDPOINT OF AN IDEALIST. 449 
the West Indies he has shown that the proportion of species which North 
America holds in common with Eurasia is 93 per cent. in the arctic regions, 
40 per cent. in the sub-arctie regions, 24 per cent. in temperate latitudes, 
and 11 per cent. in the southern portion of the continent. If we take the 
total Carex floras of the eastern and western worlds in the northern hemi- 
sphere, we find that 80 per cent. of the species held in common are arctic, 
29 per cent. sub-arctic, and 11 per cent. temperate. There are about 
150 species common to North America and Eurasia, and of these two-thirds 
are arctic and sub-arctic species. 
I am not able here to deal fully with the distribution of genera from this 
standpoint, but it cannot be doubted that the behaviour of genera common to 
both the eastern and western worlds will be intermediate between that of 
the species and families similarly distributed. This is established below in 
an analysis I have made of the list of the chief genera of the Angiosperms 
(about 3150) that is given by Dr. Willis in his ‘ Flowering Plants and 
Ferns,’ 1908. 
It has already been shown that whilst 69 per cent. of the families that 
are exclusively or mainly tropical (the subtropical regions being here 
included) oceur in both the Old and the New Worlds, the proportion for 
families that are exclusively or mainly extratropical in the northern hemi- 
sphere is 77 per cent. On the other hand, with the genera that are mainly 
or exclusively tropical the proportion found in both worlds is only 23 per 
cent. (408 out of 1781), whilst with those mainly or exclusively restricted 
to regions beyond the tropics in the northern hemisphere the proportion 
is 42 per cent. (437 out of 1045). The genera, therefore, are in their 
behaviour intermediate between the families and the species. Whilst with 
the species nearly all (80-90 per cent.) of those common to the eastern and 
western worlds gather in the high latitudes of the north, with the families 
there is but a small tendency in this direction, and reasons have been before 
given for the belief that this tendency is even smaller than is above indicated. 
With the genera the proportions common to both worlds would be, as before 
. noted, 23 per cent. for the tropics and 42 per cent. for the cooler latitudes of 
the north, and my figures suggest that north of the warm temperate region 
it would be at least 50 per cent. 
Rightly interpreted, there should be a great significance in the principle 
that the tendency to congregate in the north on the part of plants repre- 
sented in the eastern and western hemispheres is greatest and well marked 
with the species and least or non-existent with the family. The connections 
in the north belong to the later stages of the differentiating process, whilst 
the disconnections of the south date back to remote antiquity. One would 
have imagined that during the long ages that have passed the ocean-parted 
eastern and western worlds would have possessed scarcely a family in 
common, except in the north. Yet, as already shown, 69 or 70 per cent, 
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