THE NORTH SEA. 
339 
actual progress, I do not wish to be taken too literally, 
for I have myself (as herein recorded) occasionally 
witnessed it, but always in that limited and relative 
sense in which exceptions go to prove rules. 
Thus, in southern Spain, in spring, I have observed 
whole hosts of swallows to appear suddenly, as from the 
clouds. The plains, the marshes, the vineyards, or mud¬ 
flats swarm with them. One night they roost in 
thousands under your thatched eaves and in the trees 
hard by. Before the morning’s light every bird has 
gone. This may, perhaps, be explained by the assump¬ 
tion that migration is performed at altitudes determined 
by the height at which a favouring air-current is found; 
that, in this case, the air-current had ceased to flow, and 
(no other being found within available limits) the migra¬ 
tion in question was necessarily suspended, awaiting a 
renewal of the required conditions. Fortunate birds to 
have the land beneath them. Should such suspension 
occur at sea, it would involve the destruction of the 
entire host. 
Except by aid derived from the operation of physical 
laws, the nature and extent of which are unknown to 
me, and by taking advantage of “ trade-wind ” circula¬ 
tions in the upper air, I believe that migration is 
impossible for short-winged forms of sedentary habit, 
such as those above-named. But that aid, and those 
advantages, may vastly facilitate, and perhaps vastly 
accelerate, a process which is otherwise impossible. As 
to insects, there appears no evidence to prove that there 
exists any regular migration across the North Sea. 
Occasional irruptions—such as those of Colias edusct and 
of the diamond-back moth aforesaid—may be thus 
