448 MR. H. B. GUPPY : PLANT-DISTRIBUTION 
cent., occur in both the Old and New Worlds, We obtain similar indica- 
tions by also introducing the element of those families that are mainly, 
though not exclusively, either tropical or temperate. Thus, by extending 
the method employed in Tables IV. and V., we arrive at the conclusion that 
whilst 69 per cent. of the families that are mainly or exclusively tropical 
(158 in all) occur in both the Old and New Worlds, the proportion for 
families exclusively or mainly extra-tropical in the northern hemisphere 
(62 in all) is 77 per cent. (consult note at end of the paper). Under the 
circumstances the difference is small, and there is little to support the 
objeetion that the families common to thé east and the west gather in high 
northern latitudes. But it would have been enough to point out that there 
is little room for such an objection in view of the fact that the proportion of 
tropical families that are common to the eastern and western worlds (69 per 
cent.) is very close to the proportion obtained for the families of the Angio- 
sperms in the mass (70 per cent.). 
The question whether the connection by families between the Old and the 
New World is chiefly a problem of the cold regions of the north, where 
the great Ameriean and Eurasian land-masses converge, is sufficiently 
answered by the behaviour of the seven terrestrial sub-families of the 
Aracem. All of them oceur in both the eastern and the western worlds, yet 
four of them are exclusively tropical, two are distributed in both the tropical 
and the temperate zones, and only one (Calloidez) is restricted to cold 
northern latitudes. This last seems to be the only one of the seven sub- 
families that holds species common to the east and the west. 
We have now raised a very interesting point. Although the families 
common to the two worlds do not gather in the north, the species behave in 
a very different fashion. It is there that the species common to the east 
and the west mostly congregate. Thus Harshberger states that of the 364 
species of phanerogamic plants found in arctic western America, 320, or 
about 87 per cent., occur in temperate and arctic Asia; while of the 379 
species in arctic east America, 239, or 63 per cent., are also found in the 
arctic regions of Europe (‘Phytogeographic Survey of North America,’ 
pp. 311, 312; 1911). The community of species rapidly diminishes as we 
leave the north behind until we reach the tropies, where with the exception 
of a few littoral, aquatie, and marsh plants, it disappears altogether, or is 
only represented by a few plants, some of them not free from suspicion as 
regards man's agency. 
When we have two complemental families like the Myrsinaceæ and 
the Primulacesw, the first tropieal and the second temperate, it is the 
temperate family that alone displays a community of species between 
the two hemispheres. 
The manner in which the proportion of species held in common dwindles 
as we go south is well illustrated by Carer, In the author's recent book on 
