Once we accept this explanation as probable--namely, that the huanaco, in withdrawing from the herd to drop down and die in the ancient dying ground, is in reality only seeking an historically remembered place of refuge, and not of death--the action of the animal loses much of its mysterious character; we come on to firm ground, and find that we are no longer considering an instinct absolutely unique, with no action or instinct in any other animal leading up or suggesting any family likeness to it, as I said before. We find, in fact, that there is at least one very important and very well-known instinct in another class of creatures, which has a strong resemblance to that of the huanaco, as I have interpreted it, and which may even serve to throw a side light on the origin of the huanaco's instinct. I refer to a habit of some ophidians, in temperate and cold countries, of returning annually to hybernate in the saine den. A typical instance is that of the rattlesnake in the colder parts of North America. On the approach of winter these reptiles go into hiding, and it has been observed that in some districts a very large number of individuals, hundreds, and even thousands, will repair from the surrounding country to the ancestral den. Here the serpents gather in a mass to remain in a wholly or semi-torpid condition until the return of spring brings them out again, to scatter abroad to their usual summer haunts. Clearly in this case the knowledge of the hyberna-ting den is not merely traditional--that is, handed down from generation to generation, through the young each year following the adults, and so forming the habit of repairing at certain seasons to a certain place; for the young serpent soon abandons its parent to lead an independent life; and on the approach of cold weather the hybernating den may be a long distance away, ten or twenty, or even thirty miles from the spot in which it was born. The annual return to the hybernating den is then a fixed unalterable instinct, like the autumnal migration of some birds to a warmer latitude. It is doubtless favourable to the serpents to hybernate in large numbers massed together; and the habit of resorting annually to the same spot once formed, 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 instinct of returning unerringly to the ancestral 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. Probably most