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3/o 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 Chanduy 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