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delicate susceptibility would bring about a corresponding 
reversal in the direction of flight. Thus if the direction were 
against the wind in the morning it would still be against it 
in the evening. 
It seemed worth while to bring forward a few general 
considerations upon the whole subject. 
The Liberation of the Emigratory Instinct.—This subject 
was treated in Trans. Ent. Soc. Lond., 1902, pp. 462-465, as a 
further development of the views of Roland Trimen in“ South 
African Butterflies,’ vol. i, 1887, pp. 31, 32. The quoted 
sentences in the following statement were from the former 
publication. 3 
“ The instinct to emigrate * probably exists in a dormant 
state in all species liable . . . to outrun the food-supply in 
any part of their range.’ And the frequency with which the 
instinct is called into play in Pierinae is probably connected 
with their larval food-plants as well as with their power of 
multiplication; but the arresting appearance of a swarm of 
Pierines must also be taken into account as one cause of 
the numerous records. The stimulus which evokes the 
instinct must be the want of food-plant, or rather, the 
presence of food-plant useless to the future larvae. And such 
uselessness, although often caused by larval attack, may 
be also caused by drought. In either case there is aroused 
“the imperative instinct to move ’’—an instinct which often 
“further compels the individuals to move together in vast 
masses in the same direction, rather than to scatter and fly 
in all directions.’ Thus it is “ that the limits of the normal 
range of the species may be overpassed; that areas from which 
the species has been driven may be regained :—not by single 
individuals or by a very few pairs, but by immense numbers 
of both sexes... .” And, although the crowds may often 
only reach a foodless desert or the sea, the instinct, is still 
advantageous in that it removes individuals which at the time 
are a danger te the community. For the overcrowding, if 
* These movements are, for the most part, better spoken of as 
“emigration” rather than ‘‘ migration,’ because the central fact is the 
flight of vast masses of individuals owt of an overcrowded area (ibid., 
p- 465). 
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