These facts are interesting under two points of view. 1st, They show precisely the kind and extent of the influence which the states of watching and sleep exert in general on the production of heat in animal bodies ; 2d, they are remarkable in the particular instances under consideration, in this, that the differences exhi bited during the two states are extreme. It must be allowed, therefore, that those animals in which they take place must have less energetic nervous systems than other warm blooded animals. From this tendency in the animal economy, there must also be in different species a diversity rather than an equality in the degree in which the phenomena are exhibited. And this is confirmed by obser vation. Some cool to a much greater extent than others during their sleep in the summer season. They may be said severally to have just as much nervous energy as is requisite to sustain a high temperature in the summer season during their state of highest activity, i. e. during the period of watching, and no more. When the state of excitement ceases, and the collapse that follows excitement supervenes, the languor manifested is much greater than that of other animals in the same condition, and their temperature sinks in proportion. The energy possessed by hyber inating animals seems barely sufficient to enable them during the summer season to maintain a temperature of body equal to that of warm-blood ed animals in general. They subsequently pre sent another phenomenon with regard to their temperature well worthy of particular attention, although it be no more than a consequence of the first. Since it is a defect of energy in the nervous system during sleep which prevents their maintaining the degree of rapidity in the motions of circulation and respiration so essen tial in their turn to the maintenance of a tem perature of the body but little inferior to that pertaining to the state of watching in summer, how are they to preserve their temperature even during the watching state when the summer declines into autumn, and the autumn into winter ? It is evident that if they follow the general rule their respiratory and circulatory motions will be retarded with the fall of the atmospheric temperature, and this by so much the more as their nervous system shows a less degree of energy. It is even presumable that owing to the decline of atmospheric temperature in autumn, they will exhibit a temperature of body during the period of watching analogous to that which they manifest in the heat of the summer season during sleep. And this is pre cisely what happens. M. de Saissy paid par ticular attention to the state of these animals at intervals from the month of August onwards. On the 6th of August, the temperature of the air being at 22° c. (72° F.), a dormouse and a marmot marked 36,° 5 (98° F.), and a hedge hog 34° c. (93°, 5 F.) in the axilla. On the 23d September, the external temperature being 18° (64°, 5 F.), the temperature of the hedge hog was lower by 2° c., that of the marmot by 25 c., and that of the dormouse by 5°, 5 c. than it had been at the previous date. This is a considerable depression, if it be remembered that the decline in the atmospheric temperature was by no means considerable; that the air was in fact still at a point which made it be felt as warm to the generality of persons. The same individual animals examined on the 7th of November, the atmospheric temperature being 7°, presented the following state. The mar mot had lost 9°, 25 c., the dormouse 15°, 5 c., and the hedge-hog 2t°, 25 c. of their respec tive temperatures during the month of August, so that their absolute temperatures were now as follows: that of the marmot 27° (81° F.), that of the dormouse 21° (70° F.), and that of the hedge-hog 75 c. (57° F.). Here, there fore, we have several warm-blooded animals which in autumn approach very closely to the cold-blooded tribes with regard to their calorific power.
If they be next observed during the period of sleep, the relationship will be observed if possible in a more striking degree. If, during
the state of watching, they suffer such a loss of temperature as has been specified with the gradual decline of the temperature of the year, they will certainly suffer still more remarkable changes during the state of sleep, in conformity with the principles already fully developed. The sleep of these animals will also become longer and deeper in proportion as the nervous system loses its power, under the influence of the external cold, a loss which will be mani fested by a farther retardation in the motions of circulation and respiration. But what is the increasing weakness of the nervous system during sleep but a more or less marked state of torpor ? The same degree of cold con tinuing, or the degree of cold becoming gra dually greater, the disproportion as regards the animal will increase also, and will necessarily attain a term at which the torpor during sleep will become lethargic. If the external temperature goes on declining, and attains a point at which it becomes dangerous to the life of the creature, the cold, within certain limits, ought to have the power of withdrawing the animal from its state of lethargy. The excite ment which appertains to the waking period, by accelerating the motions of circulation and of respiration, will then cause the temperature of the body to rise. But if the external tem perature does not become more favourable, or if the animal finds no means of abstracting itself from its influence, it has not sufficient resource within itself and must perish.
We have seen above that the changes in the seasons produced great modifications in the constitution of warm-blooded animals in gene ral. But it were difficult to imagine any greater or more striking than those presented to us by the species which we have just named, which belong to the family of hybernating animals; changes which arise from their passing the winter months in a state of lethargy. When these animals are recalled from this state to wards the end of autumn, and during the course of the winter, they may seem to resume the characters which distinguish the vitality of warm-blooded animals in general, but they are in a very different state at this epoch from what they are in summer. Their constitution has un leraone important changes, which it is necessary to examine and appreciate exactly. These changes are inversely as those which the most perfectly constituted warm-blooded ani znals experience. These, under the influence of the increasing cold of autumn and winter, acquire new vigour, and their faculty of pro ducing heat increases in consequence. Those, on the contrary, naturally much less energetic even at the most favourable period of the year, require to be excited and supported by the high temperature of the summer or warmer months, to permit them to exhibit all their activity and strength. It is in the warm season of the year that these animals have the greatest degree of energy—Energy which has a certain duration even after the external conditions which have developed it have ceased to operate ; fur they have been as it were tempered by the continuity of favourable circumstances, espe cially of the high atmospheric temperature. This is the reason why they are so slightly affected by the diurnal variations of the warm season of the year; and even when this begins to wane, and they are no longer stimulated by the temperature proper to summer, they find sufficient energy in the store accumulated, as it were, during the fine season to enable them to resist for a time and to a certain extent the unfavourable influences with which they begin to be surrounded. These continuing, however, and even increasing, they gradually yield to their influence, and sink lethargic, till revived by the return of spring with its milder tempe rature. Their languor even augments not only with a progressively lower degree of atmospheric temperature, but with the persistence of a degree which in itself is not by any means excessive.