322 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 great advantage over others 

 of their race ; that they would be stronger 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 

 serpent, 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 ; 



