Its Origin , Migrations , and Evolutionary Tendencies. 2 1 1 
farther back, therefore, it appears clear that there is extensive migration 
along the rivers either up or down the valleys. The conditions, of course, 
are uniform, but it is worth noting that nearly all the African trees and 
shrubs have fruits and seeds that are distributed by animals, especially 
birds, and these also feed and migrate along the rivers. Succession in 
dense mesophytic forest differs in detail from that in tree veld, but it also 
follows the rivers and their tributary streams (for details see 3). The same 
applies to other subordinate types of scrub. 
We thus reach a first generalization that for trees and shrubs (mostly 
distributed by animals) composing various types of woodland (thorn veld, 
mesophytic tree veld, succulent scrub, rocky hillside scrub, dense meso- 
phytic scrub and forest) migration takes place most rapidly in a direction 
parallel to the river valleys. This, we shall see, applies chiefly to the mid- 
lands of Natal, where the river valleys are deeply cut, and many species 
seem to have difficulty in crossing the intervening ridge between one river 
valley and the next. This hypothesis has now to be tested by reference to 
the distribution of such trees and shrubs as have not spread all over the 
eastern side of South Africa. We find a large number of such relatively 
rare species confined to the coast-belt, and their lines of maximum dispersal 
and therefore presumably of their migration run parallel to the coast, but 
such species are for the most part purely tropical or slightly modified 
subtropical. 
In the Midlands, on the other hand, almost without exception, the 
rarer species have a maximum distribution parallel to the river valleys and 
ridges and at right angles to the coast-line. As an example, the Tugela 
Valley and its tributaries may be chosen. The following species are con- 
fined to it, and have not, so far as we know, as yet been able to cross into 
the next main river valley of the Umgeni farther south: Croton rividaris , 
C. menyharti , C. zambesicus , Vitex mooiensis , V. rehmanni , Heeria panicu- 
losa , Euphorbia tugelensis , Ipomaea albivenia , I. oblongata, Convolvulus 
idosepalus , Ceropegia barklyi , var. tugelensis , Fockea tugelensis , Raphionacme 
flanagani , Viscum pidchellum , V. subserratum , Pouzolzia sp., Boerhaavia 
bracteata , B. repens , Clematis glance scens, Capparis calvescens, Acacia robusta, 
Rhus excisa , Pavonia urens , Royena simii , R. scabrida , Mimusops ole if ole a, 
Ole a enervis , Strychnos dyssophila , Lippia scab err ima , Lycium pendzdinum , 
Pavetta gerrardi, Melothria maderaspatana , and others. Many of these are 
true endemics, others occur farther north, e. g. in the Transvaal but not else- 
where in Natal. In high forest, along the flanks of the ridges, the component 
species are usually more widespread, but there are examples of new endemic 
and more mesophytic species which spread most rapidly in a direction at 
right angles to the coast, e. g. Podocarpus henkelii. The river valleys and 
intervening ridges, while serving as lines of migration, also serve as barriers 
to migration across them. 
