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Bews . — The South-east African Flora : 
main, even the families represented are different. The relationship of this 
flora to the tropical is much the same as that of the great temperate flora of 
the Northern Hemisphere, and will be discussed more fully later. 
Instead of beginning with general comparisons and attempting to 
work downwards to details, it would appear more promising and more 
in accord with the usual methods of science to begin at the other end and 
study the migrations of the flora in a smaller area, afterwards extending the 
observations to larger and larger areas. As far as South Africa is con- 
cerned, the present study of plant geography in the larger sense has 
gradually developed out of detailed ecological investigations. Movements 
of species were analysed in connexion with plant succession, various lines of 
migration were traced, at first somewhat hypothetically, but the hypotheses 
were tested by an appeal to the facts of distribution, and now the final 
attempt is being made to connect up the various lines of migration so as to 
obtain a complete picture. The accepted principles of evolution in floral 
morphology have been used to check the results, but, on the other hand, the 
history of general plant migrations may be used to throw some light on 
doubtful points in evolutionary history. The most logical arrangement 
would be to begin with the tropical flora, give the reasons why the sub- 
tropical should be considered derivate, and then trace as many of the 
changes as possible. I prefer, however, to follow the order in which this 
investigation was actually carried out, since it gradually developed out 
of other work. 
River Valley Migration. 
The rivers of South-east Africa flow in a general easterly direction 
from the Drakensberg to the sea. The deeply cut river valleys are dry 
with extremes of temperature, and the vegetation is xerophytic grassland, 
thorn veld, or succulent scrub, the most extreme types being on the south 
sides of the valleys which face the sun. Along the upper flanks, especially 
on the north side, mesophytic forest develops. There are also various types 
of rocky scrub and tree veld on the flanks of the valleys. In brief, the 
various trees and shrubs (including at least a thousand species) of South- 
east Africa, composing the woodland plant communities, are, on the whole, 
associated in their distribution with the river valleys and their slopes. 
The first detailed study of plant succession was made in the thorn 
veld (1). It was afterwards seen that the other types of parkland tree veld 
were similar. The pioneers are trees. They migrate out from the river 
banks, and colonize the grassland areas, and are followed by other more 
mesophytic trees and shrubs. 
This, however, is the last step in the process of migration. The bases 
of colonization are the various river-bank woodland communities which 
extend for long distances along the river. To carry the process a step 
