452 MR. H. B. GUPPY : PLANT-DISTRIBUTION 
point of the differentiation theory its place would obviously be in the first 
grade, together with the Composite and the Araceæ. 
Having shown that the response to the cleavage of the land into two main 
masses, diverging from the north polar regions, inereases with the Angio- 
sperms as we go down the differentiating scale from the family to the 
species, being small with the family and very pronounced with the species, 
we will for a moment direct our attention to the behaviour of the great 
groups, or the cohorts, that lie immediately behind the families. As might 
have been expected, the response js even less than with the families. With 
the families about 70 per cent. ignore the cleavage. With the cohorts, on 
the other hand, 91 per cent., or 41 out of 45, are represented in both the 
eastern and western worlds (Table I.). It is noteworthy that the four 
cohorts that are exceptions to the rule hold in each case only a single family, 
the Cyelanthacez, the Leitneriacez, the Casuarinacem, and the Balano- 
psidaceze, all of them tropical, the first two belonging to the New World and 
the last twoto the Old World. They are all small anomalous families which 
have puzzled the systematist in his endeavours to place them. Together 
they represent the flotsam and jetsam of an ever-differentiating plant-world. 
The response of the families to the differentiation of the climatic zones.— 
Although the families of the Angiosperms make a relatively small response 
to the bi-cleavage of the land-surface of the globe, their behaviour under 
the stress of climatic differentiation has been very different. From the 
tables (IV. and V.) it will be seen that nearly 60 per cent. of them are 
exclusively or mainly tropical, about 30 per cent. exclusively or mainly 
temperate, and about 10 per cent. fairly divided between the tropical and 
the temperate zones, all the regions outside the sub-tropies being regarded as 
temperate. Since the differentiation theory postulates an early age in the 
history of the Angiosperms when primitive generalized types ranged 
the globe and uniform climatic conditions prevailed, the later ages being 
occupied with the differentiation of types in response to the diversification 
of climate, it follows that the results for families above given represent a 
particular stage in the detachment or individualization of temperate floras. 
When we look to the future and ask ourselves what will be the ultimate 
result of this gradual detachment of the temperate from the tropical floras, 
we shall be obliged to confess that there is little more to expect now. We 
might have looked far ahead to an age when the tropical and temperate 
floras would be sharply differentiated, an age when the world would be held 
by complemental families representing the independent expression of tropical 
and temperate conditions on the same type. But the influence of climatic 
differentiation is largely played out. Nature in the development of new 
forms seems to have mainly exhausted her efforts during the Upper 
Cretaceous period. That which has happened since has been principally 
the effect of the differentiation of ancient types in response to the progressive 
