450 MR. H. B. GUPPY : PLANT-DISTRIBUTION 
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of the exclusively and mainly tropical families are common property of the 
Old and the New Worlds. All the influences that have been in operation 
in a differentiating world during an incalculable period of time have in a 
general sense not materially defaced the primitive family type, and the 
wonder is not that the differentiating agencies have done so much but that 
in this respect they have effected so little. 
Yet, as we have seen, about 30 per cent. of the families do respond to 
the bi-cleavage of the land represented in the American and Eurasian 
hemispheres. (This applies, of course, only to the families in the mass, the 
proportion, as shown in a note to Table III., being much smaller, if we 
regard only the principal families.) But the differentiation, or the break-up, 
of the original family-type has proceeded far more rapidly in the New than 
in the Old World. In its development of new families the western hemi- 
sphere displays for its size nearly twice the capacity that is exhibited by 
the eastern hemisphere. Of the eighty residual families that are restricted 
either to one hemisphere or to the other (see Tables I., II.), one would 
have expeeted the Old World to possess by far the greater number, since 
the land-areas of the two hemispheres stand to each other in proportions 
exceeding two to one (O.W. 35; N.W. 15; based on data given in 
Whitaker's Almanack, 1917, p. 101, the polar regions being excluded, 
Australia being included in the Old World). But, to one's surprise, the 
difference in the number of families peculiar to each is relatively small, 
45 being appropriated by the eastern and 35 by the western hemisphere. 
There will subsequently be occasion to mention this point again in associa- 
tion with another remarkable contrast presented by the New World with 
regard to the Old World. 
[One may take this opportunity to observe that the excess in families in 
proportion to its area held exclusively by the New World is apparently not 
exhibited to the same degree by the genera. Of the 3150 genera of the 
Angiosperms named by Willis in his * Flowering Plants and Ferns’ (1908), 
the Old World appropriates 47 per cent. and the New World 25 per cent., 
about 28 per cent. being held in common. Of 529 genera belonging to 
42 families dealt with in the ‘Pflanzenreich’ series, 51 per cent. are peculiar 
to the Old World, 32 per cent. to the New World, and 17 per cent. are 
common to both. The difference between the two worlds with regard to 
their peculiar families and genera may be thus expressed. With the 
families the difference would be as 9 (O.W.) to 7 (N.W), but with the 
genera as 9 (O.W.) to 5 (N.W.).] 
We have already observed that as many as 70 per cent. of the families do 
not respond to the great bi-cleavage of the land-surface of the globe, 
distributed as they are in both the eastern and western hemispheres. The 
response becomes greater and greater as we proceed down the seale from 
the family to the species, Thus the proportions common to the two 
