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Bews. — Plant Succession and 
the mountain ranges to the Drakensberg (detailed lists have been given 
in some of my former papers ( 1 , 3 , 4 )). As far as the eastern region 
is concerned they are ‘ wides yet they have an extremely restricted 
range in the eastern region. It may be emphasized that these are not 
a few isolated exceptions, for the numbers are considerable. 
Again, if we take Natal, we find that on the tropical coast-belt there 
are a large number of tropical species which have a wide range to the north 
through the tropics. Out of a total of about 1,500 species 545 are 
recorded also in the ‘Flora of Tropical Africa 5 , or 36*3 per cent. These 
extend north of the Tropic of Capricorn. Now practically all those 545 
species have a very narrow range in Natal, being confined to the frost- 
free localities of the narrow coast-belt, yet they are all ‘ wides ’ as far 
as Natal or South Africa is concerned. If we attempt to deal with 
Natal as a whole on the same lines as those adopted by Willis for Cey- 
lon the results obtained are utterly different. The reason of course is 
obvious. Natal is a country with very diverse climatic conditions. One 
set of ‘ wides 5 from the south-west have a restricted distribution along 
the mountains of Natal, another set of ‘ wides ’ from the tropics have a 
restricted distribution along the coast-belt of Natal. 
This line of criticism has to a large extent been met by Willis, 
though, as he admits himself, it is unfortunate that he did not do so in 
his earlier papers. In his reply to criticism by Ridley ( 12 ) he states 
( 17 , p. 191), ‘I much regret that in my Ceylon papers I did not make 
sufficiently clear the various conditions that might modify the action of 
my age and area rule. Partly this was because I thought that many 
were obvious, and partly because I was thinking more of making the 
law itself clear. 1 He goes on later to give a list of such modifying 
causes as follows ( 17 , p. 206) : 
1. Chance (the operation of causes as yet not understood) ; 
2. Action of man in opening up a country, cutting of forest, exploring, 
making fires, &c., &c. ; 
3. Interposition of barriers, such as mountains, broad rivers, desert, 
arms of the sea, sudden changes of climate from one district to the next, and 
the like ; 
4. Geological changes, especially if involving change of climate ; 
5. Serious changes of climate ; 
6 . Natural selection ; 
7. Local adaptation ; 
8. Dying out of occasional old species ; 
9. Arrival of a species at its climatic limit ; 
10. Density of vegetation upon the ground at the time of arrival 
of a species ; 
11. Presence or absence of mountain chains ; 
