370 
NOTES OF A BOTANIST 
CHAP, 
Upon them. There are also cattle- farms ; and 
around the wells from which water is drawn and 
served to the cattle the same weeds are continually 
springing up ; while the seeds, even of those that 
grew on the desert, remain embedded in the sand 
and retain their vitality during all the years of 
drought. When the rains come on, therefore, they 
cause, as it were, a unilateral development of the 
vegetation from the forest across the open grounds, 
and a corresponding expansion of the insect-life 
which breeds and feeds upon it. 
Results the same in principle, but diverse in 
mode, would take place under different local circum- 
stances. Thus, if we suppose an oasis in the midst 
of a desert exposed to the same exceptional access 
of moisture as the desert of Chandiiy with its forest 
skirt, there would be generated an extension of 
organic life radiating outwards in all directions. 
Besides the migrations above recorded, I have 
many times in South America seen butterflies flying 
across rivers so wide that it is impossible to suppose 
they could be guided by any indication of sight or 
smell. Animals of higher organisation and stronger 
reasoning powers would probably turn aside along 
the shore of the river or ocean in quest of food for 
themselves and their offspring ; but there are 
plainly cases where frail little creatures, such as 
butterflies, must go straight forward at a venture, 
and either attain their object or perish. 
Migrating Ants 
The movements of Ants registered in my journal 
are (as may be supposed) chiefly such as were 
