Plant Distribution in South Africa, 289 
12. Relative width of the union between the country of departure and 
that of arrival. 
He states further : ‘ Some of these causes probably come into action in 
almost every single case of any individual species, though upon large 
numbers and in the long run they cancel out. All might have been 
covered had I added to my tentative statement of the law the phrase 
“ so long as conditions remain constant ”, or words to that effect.’ 
The chief causes which apply to Natal are Nos. 3 and 9 as given above, 
but the numbers involved are so large that there can be no question of their 
cancelling out, nor does a phrase such as ‘ so long as conditions remain con- 
stant ’ cover everything. It is not a question of present conditions re- 
maining constant so much as that the law of age and area applies only 
to regions where conditions are fairly uniform, as in Ceylon and New 
Zealand. It is only in his latest paper on * The Flora of Stewart Island ’ (21) 
that Willis expressly recognizes this, but it appears to me to be the fact 
which above all others requires emphasis. In the same paper ( 21 , p. 23) 
Willis draws a distinction between ecological and taxonomic distribution, 
and states that it is to the latter that age and area refers. ‘ The two lines 
of work ’, he says, ‘ are really very distinct, and have comparatively little 
overlap. The plants are locally distributed, within the area which is 
assigned to them by the passage of time, in accordance with their reactions 
to the various ecological factors which are operative there. Ecology simply 
seems to make what it can, so to speak, of the floras with which it is 
provided by the mere action of phylogenetic descent and of time.’ 
Whether or not the two lines of work are as distinct as Willis believes, 
it seems clear that ecological principles must modify the action of the age 
and area rule, and the object of this paper is to see how far plant distribu- 
tion in South Africa can be explained on purely ecological lines. Certain 
general principles are put forward which, so far as I am aware, have not 
been expressly stated before. The very fruitful concept of plant succession 
is entirely ignored by Willis, yet it throws light on many facts which other- 
wise would remain obscure. 
Widely Distributed Species in South Africa. 
The fact that the only general ‘ flora ’ of South Africa, the ‘ Flora 
Capensis ’, is still incomplete, and more than half the total number of 
families were dealt with in the first three volumes, which were published 
nearly sixty years ago, at a time when Natal, the Orange Free State, and 
the Transvaal were almost entirely unexplored, makes any systematic 
investigation into plant distribution in South Africa a matter of considerable 
difficulty. We have, however, two or three fairly complete check-lists, and 
for our present purpose it is sufficient to pick out the species that are 
common to Natal on the eastern side, and the Cape Peninsula on the 
