174 THE BOOK OF A NATURALIST 
last being the fear of an enemy which the young 
learn from their parents or other adults they 
associate with. Fear is contagious; the alarm of 
the adults communicates itself to the young, with 
the result that the object that excited it remains 
thereafter one of terror. Not only in this matter 
of snakes and monkeys, but with regard to other 
creatures, Darwin lays it down that in the higher 
vertebrates the habit of fear of any particular 
enemy quickly becomes instinctive; and this false 
inference has been accepted without question by 
Herbert Spencer, who was obliged to study animal 
habits in books, and was consequently to some 
extent at the mercy of those who wrote them. 
It is frequently stated in narratives of travel 
in the less settled portions of North America that 
all domestic animals, excepting the pig, have an 
instinctive dread of the rattlesnake; that they 
know its whirring sound, and are also able to smell 
it at some distance, and instantly come to a dead 
halt, trembling with agitation. The fear is a fact; 
but why instinctive? Some time ago, while reading 
over again a very delightful book of travels, I 
came to a passage descriptive of the acute sense 
of smell and sagacity of the native horse; and the 
writer, as an instance in point, related that fre- 
quently, when riding at a swift pace across country 
on a dark night, over ground made dangerous by 
numerous concealed burrows, his beast had swerved 
aside suddenly, as if he had trod on a snake. His 
sense of smell had warned him in time of some 
