illustrated by the South African Flora . 3 
In his latest book (26), however, Guppy accepts Thiselton-Dyer’s own 
views. He also elaborates the ‘ theory of differentiation ’, applying it first 
to families which, he states, tend to fall into two groups, the primitive and 
derivative, the first world-ranging, and the second restricted in their area. 
Differentiation and decrease of range go together. He goes on to apply 
the same principle ‘ in the tribe, in the genus, in the species, and in the local 
race or variety ’. He takes the Geraniales, Geraniaceae, Geranieae, 
Geranium, sections of the genus Geranium , as examples, and quotes the 
work of Andrews on the development of the families Myrtaceae (1) and 
Leguminosae (2) in Australia. 
Guppy further points out that, ‘ if the differentiation hypothesis is 
correct, no natural order could have been developed on the lines implied by 
the Darwinian theory, which, as interpreted in recent works, begins with 
the variety and terminates with the order, a process that reverses the usual 
methods of Nature ’. ‘ Yet such a process’, he says, ‘as is there implied is 
common enough in the plant world, but it accounts not for the natural 
orders but for the oddities of plant forms.’ ‘ It is here termed a specializing 
process in contrast with that of differentiation ; but it is the differentiating 
process that has been the principal determining cause of diversification in 
plants ’. There is probably much truth in this theory, though, taking into 
consideration the palaeontological record, it is doubtful whether it contains 
all the truth. 
Guppy goes on to state that the differentiation theory could of itself 
explain distribution, if the land areas of the world were continuous and not 
affected by unstable climatic conditions, and he deals with the factors that 
modify its operation. 
The theory of southward migration favoured by Thiselton-Dyer and 
Guppy permits«of postulating the permanency of the general configuration 
of the land -masses of the earth’s surface, and is so far in agreement with the 
views of Darwin and Wallace. The southern end of South Africa is a ‘ cul- 
de-sac into which the species have poured and from which there is no escape ’, 
hence the extraordinary congestion of species there. Again, as Guppy 
remarks (26), ‘ if it can be shown, as undoubtedly the general trend of the 
facts of distribution does show, that the divergence of plant types responds 
to the divergence of land-masses from the north, and that dissimilarity is 
intensified with distance from that pole, any evidence for a Tertiary 
Antarctic centre for the flowering plants would be discounted in advance 
Thiselton-Dyer and Guppy agree that the centrifugal dispersion of species 
from the north, which all acknowledge took place during the last Ice period, 
has been repeated often during the course of geological time. 
‘ At a time when a genial climate prevailed over the northern or land 
hemisphere, the plants now represented in type in the warmer latitudes 
occupied the regions beyond the Arctic Circle. When this period gave 
