But, independent of these instances of submersion, as it is termed, which we regard as purely accidental, there have been many instances of actual torpidity observed. Swallows, if we may credit the testimony of many who have been eye-witnesses of the fact, are often found during the winter season in a torpid state in their old nests, and in the crevices of old buildings. The belief of this kind of torpidity is very common in many parts of Scotland, and can scarcely be supposed to have originated from any other cause than the occurrence of the fact.
But besides the occurrence of the torpidity of the swal low, Bewick relates an instance of the same condition be ing observed in the cuckoo. 41 A few years ago a young cuckoo was found in the thickest part of a close whin-bush. When taken up it presently discovered signs of life, but was quite destitute of feathers. Being kept warm, and carefully fed, it grew and recovered its coat of feathers. In the spring following it made its escape, and in flying across the river Tyne it gave its usual call."— Brit. Birds, 1. Introd. xvii.
There is a still more decided example of torpidity in birds recorded by Mr Neill, in his Tour through Orkney and Shetland, as having been observed in the case of the land-tail, or corncrake, as it is called in Scotland. "I made," says he, " frequent inquiry, whether corncrakes had been seen to migrate from Orkney, but could not learn that such a circumstance had been observed. It is the opinion of the inhabitants, indeed, that they are not able to undertake a flight across the sea. Mr Yorston, the farmer at Aikerness, further related a curious fact, rather leading to the conclusion that they do not migrate. In the course of demolishing a lull-dyke, (i. e. a mud wall,) at Aikerness, about midwinter, a corncrake was found in the midst of the wall. It was apparently lifeless ; but, being fresh to the feel and smell, Mr Yorston thought of placing it in a warm situation, to see if it would revive. In a short time it began to move, and in a few hours it was able to walk about. It lived for two days in the kitchen, but would not eat any kind of food. It then died, and be came putrid.—I do not assert that this solitary instance ought to be regarded in any other light than as an exception to the general rule of migration, till further observation has determined the point." These are the only instances, with which we are ac quainted of actual torpidity having occurred among the feathered tribes. They seem calculated to remove all doubt as to the fact, while they point out to us the nu merous resources of nature in extreme cases to preserve existence. Thus when birds from disease or weakness, or youth, are incapable of performing the ordinary migra tions of their tribes, they become dormant during the win ter months, until the heat of spring restores to them a sup ply of food and an agreeable temperature.
Hitherto we have been considering the torpidity which warm-blooded animals experience. Several cold blooded
animals observe a similar mode of hybernation.
The period of the year at which REPTILES prepare for this state of lethargy varies in the different species. In general, when the temperature of the air sinks below the 50th degree of Fahrenheit, these animals begin their win ter slumbers. They adopt similar precautions as the mam rnalia, in selecting proper places of retreat, to protect them- from their enemies, and preserve them from sudden alternations of temperature. Those which inhabit the wa ters sink into the soft mud, while those which live on the land enter the holes and crevices of rocks, or other places where the heat is but little affected by changes in the tem perature of the atmosphere. Thus provided, they obey the impulse, and become torpid.
As the temperature of these animals depends on the sur rounding medium, they do not exhibit any peculiarities with respect to it. When the air is under these ani mals become torpid, and suffer their temperature to sink as low as the freezing point. When reduced below this, either by natural or artificial means, the vital principle is in danger of being extinguished. In this torpid state, they respire very slowly, as the circulation of the blood can be carried on independent of the action of the lungs. Even in a tortoise kept awake during the winter by a genial temperature, the frequency of respiration was observed to be diminished.
The circulation of the blood is diminished, in propor tion to the degree of cold to which these torpid reptiles are exposed. Spallanzani counted from eleven to twelve pulsations in a minute in the heart of a snake at the tem perature of 43°, whose pulse in general in warm weather gives about thirty beats in the same period. Dr Reeves made some very interesting experiments on the circula tion of the toad and frog. " I observed," he says, " that the number of pulsations in toads and frogs was thirty in a minute, whilst they were left to themselves in the atmos phere of which the temperature was ; when placed in a medium cooled to 40°, the number of pulsations was re duced to twelve within the same period of time ; and when exposed to a freezing mixture at 26°, the action of the heart ceased altogether." The powers of digestion are equally feeble during tor pidity as those of respiration or circulation. Mr John Hunter conveyed pieces of worms and meat down the throats of lizards when they were going to their winter quarters, and, keeping them afterwards in a cool place, on opening them at different periods, he always found the substances, he had introduced, entire, and without any alteration ; sometimes they were in the stomach, at other times they had passed into the intestines, and some of the lizards that were allowed to live, voided them toward the spring entire, and with very little alteration in their struc ture.