442: MR. H. b. GUPPY : PLANT-DISTRIBUTION 
differentiated in response to the progressive diversification of conditions 
originally uniform, a world in which the family differentiated into tribes, 
the tribes into genera, and the genera into species. But on perceiving that 
such a theory could only explain distribution where a continuous land-mass, 
not affected by unstable climatic conditions, was concerned, he came to 
recognize that the operations of the differentiating agencies had been largely 
controlled and directed by the divergence of the two great land-masses from 
the north, a control in its turn influenced by the secular fluctuations of 
climate and by the barriers that lay across the lines of migration. It may 
be stated that the progressive differentiation of floras in response to diversi- 
fication of conditions has long been recognized. The era of world-wide 
floras, as Mr. C. Reid well puts it (‘Encyclopedia Britannica, edit. 10, 
vol. xxxi. p. 432), began to pass away after the Cretaceous age, and from that 
period onward plants have responded more and more to the differentiation 
of conditions and have arranged themselves more and more according to 
geographical boundaries and climatic zones. The general trend of events in 
later ages is sufficiently indicated by the frequent application to these early 
types by various writers of such epithets as “comprehensive,” * generalized,” 
* mixed," * synthetic," ete. 
The views long advocated by Thiselton-Dyer brought about the first 
modification in the original interpretation of the theory, and the writer came 
in this way to lay stress on the point that the mingling of the floras of the 
eastern and western worlds might be regarded as the result of the successive 
migrations to and from the north polar region under the stress of climatic 
changes. The third stage was reached when he realized as a result of the 
statistical treatment of the subject, which is dealt with in later pages, that 
there was much in the distribution of the larger groups which the mixing of 
the eastern and western floras in the north polar regions would not explain. 
Though true of the smaller groups, as with species, the principle that the 
community between the Old and the New World is an affair of the north 
did not materialise with respect to families. On the contrary, it appeared, 
with regard to families of the first rank and the groups behind them, that 
the main features of. distribution would have been much the same as they 
now are if the land of the globe had been gathered into a single mass. 
Thus the author came to distinguish between the larger and smaller groups 
in the response made to the great bi-cleavage of the land-surface of the 
globe, and to restrict the influences of the existing relations of land and sea 
to the smaller groups, as in the case of genera and species. This led him to 
perceive that if the differentiation hypothesis was valid the families and the 
larger groups behind them had. not only ignored the bi-cleavage of the land- 
mass of the globe, but had been developed under conditions very different 
from those in which their genera and species had been produced. 
The independent behaviour of the great families with respect to existing 
