Nov. 1905. | ACACIAS IN VARIOUS PLACES. 133 
long after their unprotected neighbours have withered away. 
So that the manure of grazing animals favours the change 
from pioneers to thicket or from thicket to woods. 
I shall mention two cases in which such differences have 
been brought about. 
The guanaco in South America has the curious habit of 
depositing its droppings in one locality for long periods. 
These spots are conspicuous on account of their rich green 
vegetation amidst the surrounding desolate and burnt-looking 
steppe. 
Then also, in some parts of India, conspicuous green 
patches may be noticed on the burnt-up, brownish-yellow 
hillsides. These are where the natives have folded their 
tlocks, and the more vigorous growth due to manure has 
prevented drought from having its usual effect. 
I could mention similar cases in this country. 
These considerations are sufficient, I think, to show that 
an association is not a stationary organism susceptible of 
exact definition, but that it is always trying to extend its 
borders, It is probably also, like a species or like an animal 
during its life, perpetually changing with varying conditions, 
or, if it refuses to adapt itself, being suppressed by something 
better able to stand the new conditions. 
Since writing the above paper, a remarkable confirmation 
of my views has appeared (‘Geographical Journal,” Dec. 
1905, p. 670). 
“This year the whole distance between Algeria and the 
Niger has been traversed by one traveller, M. E. F. Gautier, 
whose excellent studies on the physical geography of the 
North Saharan borderlands have been frequently alluded to 
in the Journal. The explorers set out from Tuat on 12th May 
1905. Gao, on the Niger, was reached on 3rd August. A 
short account of the journey, based on a communication to 
“Le Temps,” appeared in the October number of “La Géo- 
graphie.” For the last 375 miles before reaching the Niger, 
the traveller crossed an unbroken steppe, covered with a 
widely spaced forest of mimosas, between which a fine grass 
grows. This M. Gautier considers as characteristic of the 
southern border of the desert from the Atlantic to Egypt.” 
