342 The Naturalist in La Plata. 
tagious, like fear, that communicates itself, quick 
as lightning, from one to another until all are in a 
panic, and like the joyous emotion that impels the 
members of a herd or flock to rush simultaneously 
into play. 
Now, it is a pretty familiar fact that animals acting 
instinctively, as well as men acting intelligently, 
have at times their delusions and their illusions, 
and see things falsely, and are moved to action by a 
false stimulus to their own disadvantage. When 
the individuals of a herd or family are excited to a 
sudden deadly rage by the distressed cries of one 
of their fellows, or by the sight of its bleeding 
wounds and the smell of its blood, or when they see 
it frantically struggling on the ground, or in the 
cleft of a tree or rock, as if in the clutches of a 
powerful enemy, they do not turn on it to kill but 
to rescue it. 
In whatever way the rescuing instinct may have 
risen, whether simply through natural selection or, 
as is more probable, through an intelligent habit 
becoming fixed and hereditary, its effectiveness 
depends altogether on the emotion of overmastering 
rage excited in the animal—rage against a tangible 
visible enemy, or invisible, and excited by the cries 
or struggles of a suffering companion ; clearly, then, 
it could not provide against the occasional rare 
accidents that animals meet with, which causes them 
to act precisely in the way they do when seized or 
struck down by an enemy. An illusion is the result 
of the emotion similar to the illusion produced by 
vivid expectation in ourselves, which has caused 
many a man to see in a friend and companion the 
