animating the monotonous
splendour of the ocean; they arrive as soon as a vessel is one or two
days' journey from land. Henceforth they do not leave her, flying
behind and plunging in her wake; they profit by the disturbance
produced by the gigantic machine to capture the stunned fishes.

On land exactly the same kind of chase is carried on by Rooks, Crows,
and Magpies, who follow the plough to seize the worms which the
ploughshare turns up in the open earth. In autumn they cover the
fields, animated and active, pilfering as the furrow is hollowed out.

Certain rapacious birds who are awkward in hunting, especially Kites,
make up for their lack of skill by audacious impudence. Constantly on
the watch for better hunters like the Falcon, they throw themselves on
him as soon as he has seized his prey. The proud bird, though much
more courageous, stronger, and more skilful than these thieves,
usually abandons the prey either because the burden embarrasses him in
the struggle, or else because he knows that he can easily find
another. These highway robbers of the air often unite to gain
possession of a prey already taken and killed, and ready to be eaten.
A handsome Falcon of the Southern States of North America, the
Caracara Eagle (_Polyborus cheriway_), frequently steals fish from the
Brown Pelicans on the coast of Texas. When the Pelicans are returning
from their expeditions with pouches filled with fish, the Caracaras
attack them until they disgorge, and then alight to devour the stolen
prey. They do not attack the outgoing birds, but only the incoming
ones, and they wait until they reach the land (so that the contents of
the pouches may not fall into the water) before pouncing on them.[25]

[25] Bendire, _Life Histories of North American Birds_,
p. 315.

Among other animals a habit has been formed from some special
circumstance. As an extreme case in this group we meet with parasites
of whom some cannot live outside a particular nest, and are even
absolutely transformed by this kind of life. But between these and
independent hunters there are an extreme number of intermediate
stages, of which it is sufficient to mention a few.[26]

[26] For a discussion of this subject, see P. van Beneden,
_Commensaux et Parasites_, Paris, 1875.

The _Fierasfer_, a little fish of the Mediterranean, installs himself
in the respiratory cavity of a Holothurian; he does not live at the
expense of his host's fles