XXIII 
ANIMAL MIGRATIONS 
369 
In April swarms of butterflies and moths appeared, 
coming from the east, sucking the sweets of the 
newly-opened flowers, and depositing their eggs on 
the leaves, especially of a Boerhaavia and of a 
curious Amaranth (Frohlichia, sp. n.) not unlike our 
common Ribgrass in external aspect — until cater- 
pillars swarmed on every plant. New legions 
continued to pour in from the east, and finding the 
field already occupied, launched boldly out over the 
Pacific Ocean, as Magellan had done before them, 
there to find a fate not unlike that of the adven- 
turous navigator/ No better luck attended most of 
the offspring of their predecessors, especially those 
who fed on the Boerhaavia, which was much less 
abundant than the Frohlichia. The shoal of cater- 
pillars advanced continually westward, eating up 
whatever to them was eatable until, on nearing the 
seashore and the limit of vegetation, I used to see 
them writhing over the burning sand in convulsive 
haste to reach the food and shelter of some Boer- 
haavia which had haply escaped the jaws of preceding 
emigrants ; but, failing this, thousands of them were 
scorched to death, or fell a prey to the smaller sea- 
side birds, to whom they were doubtless a rare 
dainty. 
The explanation of this continual westward move- 
ment is not difficult. A few leagues inland, instead 
of the sandy coast-desert with here and there a tree, 
we find woods, not very dense or lofty, but where 
there is sufficient moisture to keep alive a few 
remnants of the above-mentioned herbs all the year 
round, and doubtless also of the insects that feed 
^ Here also the course attempted to be steered by the insects was across the 
strong southerly breeze that was blowing. 
VOL. II 2 B 
