454 MR. H. B. GUPPY : PLANT-DISTRIBUTION 
questionable whether any truly natural family comparable with the great 
families has been developed through the changes inducing the xerophytic 
organization. They might disguise them, as in the case of the Cactacere 
and of some of the Euphorbiacem ; but the essential floral characters were 
produced in pre-xerophytic times. A family in its truest sense is born and 
not made. 
It may here be added that the process of detachment of temperate families 
from the tropies has not been uniform in the two hemispheres, the east and 
the west, the tendency to the differentiation or detachment of temperate 
families in the Old World being far more marked than in the ease of the 
New World. Thus, with the exclusively Old World families 44 per cent., 
or 20 out of 45, are restricted to the tropies, using that term as including 
the sub-tropies ; whilst with the exclusively New World families the pro- 
portion is as high as 77 per cent. or 27 out of 35 (Table IIT.). Thus it 
also appears that the tropics of the American continents actually possess a 
greater number of peculiar families than those of the Old World. This 
feature of American plant-distribution is to be associated with another 
feature, already alluded to and illustrated in the same iable, the New World 
in its entirety owning nearly as many peculiar or endemic families as the 
Old World, 35 against 45, though barely half its size. In the New World, 
therefore, there has been not merely a greater development of families, but 
a greater segregation of such families within the tropics. In other words, 
although the process of detachment of temperate families from the tropics 
has been far less effective in the New World than in the Old World, the 
differentiation of new families in the American tropics has been far greater. 
This is one of the lessons supplied by the western hemisphere when treated 
statistically from the standpoint of the differentiation hypothesis. | Reference 
has before been made to the contrast in behaviour of the genera in one of 
these respects. | 
Before quitting the subject of the influence of the differentiation of the 
climatic zones on the development of the families, a few remarks may be 
devoted to the numerical distribution of the families of the Angiosperms 
in the north and south hemispheres, which is illustrated in Tables VIII., 
IX. X. The matter cannot be discussed at any length here ; but it may be 
observed that the numerical apportionment is much as though the land-areas 
in the two hemispheres were approximately the same in extent. Yet within 
the limits of vegetation there must be at least 24 times as much land in the 
northern as there is in the southern hemisphere. The differentiation of 
families has thus been far more active in the south than in the north, a 
result that might be attributed to the much greater isolation of the southern 
land-masses. 
The larger plant-groups behind the fumilies.—W e have already dealt with 
the cohorts in connection with the families, and it has been shown that as 
