
860 THE AMERICAN NATURALIST. [Vor. XXXVI. 
dass auch die Bewegung der Nachtschmetterlinge meteorolo- 
gischen Beeinflussungen unterworfen sind. Diese Ansicht 
stiitzt sich auf wiederholte Beobachtungen, nach welchen die- 
selben unter gleichen Bedingungen wie die Vögel, und fast 
immer zusammen mit diesen in ostwestlicher Richtung hier 
vorbeiziehen "; which may be the key to the whole question. 
As far as I am aware, no barometric observations have been 
made directly on the subject of insect migration (15). 
That a feeling of hunger is not alone a sufficient cause for 
some of the direct flights recorded, is shown by the fact that 
butterflies often continue in their path, over the very flowers 
they use for food, only a chance one stopping on the way 
(3, P. 509). Also, if they simply spread out to find food, 
the progression would be diffuse, directions inclined to be 
random, and the advance a gradual one, as is the case with the 
spring “ dispersal through isolated individuals" of Danais (5) 
which go north as the spring advances, various generations 
taking part in the advance. 
In many instances the direction of flight has not been noted, 
but in Europe the general tendency seems to be from east to 
west. In our country, however, no such law appears to hold. 
More attention has been paid to the consideration of whether the 
animals, chiefly butterflies, flew against or with the wind, some 
holding that the latter feat was impossible. That it is at least 
possible, has been shown by some direct observations (6, 7). 
It has been suggested that, as in some cases the swarm was 
composed of individuals of one sex (males), the flight origi- 
nated in a search for mates; but as the absence of females 
can be explained in some instances by their wingless condition 
(8, 1900, p. 13), while in most observations the sexes were nearly 
equal in number, this cause may apparently be ruled out. 
If they are then not necessarily merely drifted by the wind, 
or impelled by immediate hunger or by the sexual instinct, 
why do they follow such direct routes, fly with such apparent 
aim, and often repeat the flight at intervals? Walker attrib- 
utes it to *a propensity to migrate” (8, 1901, p. 353), thus 
throwing it back upon hereditary tendencies, while Kefer- 
stein (4) adds to over-production and some aid from wind, 

