32 2 The Naturalist in La Plata, 
we can imagine that the individuals — perhaps a 
single couple in the first place — frequenting some 
very deep, dry^ and well-sheltered cavern, safe from 
enemies, would have a Q:reat advantao’e over others 
of their race : that tliev would be strong^er and 
increase more, and spread during the summer 
months further and further from the cavern on all 
sides ; and that the further afield they went the 
more would the instinct be perfected; since all 
the young serpents that did not have the in- 
stinct of returning unerringly to the ances- 
tral refuge, and that, like the outsiders of their 
race, to put it in that way, merely crept into the 
first hole they found on the approach of the cold 
season, would be more liable to destruction. Pro- 
bably most snakes get killed long before a natural 
decline sets in ; to say that not one in a thousand 
dies of old age would probably be no exaggeration ; 
but if they were as safe from enemies and accidents 
as some less prolific and more highly-organized 
animals, so that many would reach the natural term 
of life, and death came slowly, we can imagine that 
in such a heat-loving creature the failure of the 
vital powers would simulate the sensations caused 
by a falling temperature, and cause the old or sick 
serjoent, even in midsummer, to creep instinctively 
away to the ancient refuge, where many a long 
life-killing frost had been safely tided over in the 
past. 
The huanaco has never been a hybernating ani- 
mal ; but we must assume that, like the crotalus of 
the north, he had formed a habit of congregating 
with his fellows at certain seasons at the same spot ; 
