It seems impossible, therefore, to doubt from what precedes, that size is not an element which has much influence in the particular direction we are considering. We have seen that with a decrease in the size of adult Mam inalia the circulatory and respiratory motions were progressively accelerated, and that by this means the disadvantages as regards cooling in consequence of a smaller relative size of the body, are in some measure compensated, some times, indeed, we have seen the balance in clined the other way, and the greater rapidity of the motions more than compensate for the diminished size of the body. Great rapidity of the respiratory and circulatory motions may co-exist with other organic conditions having an opposite tendency as regards temperature ; and, according to the relations of these, and as the one or the other predominates, we may have two different states of temperature in early life. This proposition is even made ap parent when we compare the constitution in early youth and in adult age. In early life the celerity of the motions has led to the belief that all the functions of nutrition were pecu liarly active. But strength or energy is not always an accompaniment of simple celerity ; on the contrary rapidity is generally indicative of absence of power. It is quite true that in early life not only are circulation and respira tion, but digestion, assimilation, and growth likewise, much more rapid than in the adult state. But does it follow from this that the materials of the blood are elaborated in the same degree of perfection, or that the products of the action and contact of this fluid, the various tissues, &c. of the body, are all as com pletely formed ? Everything conduces to make us believe that the reverse is the case. If on the one hand rapidity of movement be a cha racter of early life, weakness is a feature still more manifest. If the nervous system there fore, although acting rapidly, is less energetic, in the same proportion there may be an age at which the influence of this weakness on the production of heat may be manifest. And, as the weakness is greater as the being is younger, it is in the very earliest periods of independent existence that this relation must be inves tigated. Now such a relationship does actually exist, although an opinion to the contrary had always been entertained until direct experi ments settled the question definitively. 1 hese experiments were performed by the writer, and a summary of them is here given. If the temperature of new-born puppies lying beside their mother be taken, it will be found from one to three degrees inferior to that of the parent. The same thing obtains in regard to the young of the rat, the rabbit, the guinea-pig, &c. and is probably universal among the Mani Inaba. Among Birds the same circumstance presents itself in a still more marked degree. If they be taken out of the nest in the first week or even fortnight of their existence, the difference of temperature extends to from 2° to 5° c. between the young and the parents. The fact has been ascertained in regard to the sparrow, the swallow, the martin, the sparrow hawk, the magpie, the thrush, the starling, &c. &c., and is probably, as among Mammal ia, universal. Whence we may conclude that the phenomenon is general as regards warm-blooded animals. We might have taken it for granted that man was comprised within the category, but it is just as well to have the assurance that lie forms no exception to the law, that he has no peculiar privilege in this respect. To have a precise term of comparison, the temperature or twenty adults was taken at the same time, the thermometer being applied in the axilla. The temperature of these twenty persons varied between 35°, 5 and c. (96° and 99° F.); the mean term was therefore 36°, 12 (97°F.). The temperature of ten infants varying from a few hours to two days in age, ascertained in the same manner,varied between 34° and 35° 5 C. (93°, 5 and 96° F.). The mean was there fore 34°, 75 c. (about 94°, 5 F.). There was consequently a difference of nearly two degrees between the temperature of the adult and of the newly born babes. Man is therefore proved to be subjected to the same law here as ani mals having warm blood in general, the young of which, so far as they have been examined, and we may presume universally, are inferior in temperature to their parents.
There are, therefore, two periods in youth at which the bodily temperature differs from that of the adult age. These may be distinguished as the first and second periods of infancy or youth. The first extends from birth to an in definite period, but which is nearer or more remote from the period of birth in difkrent cases. The second is included between the fourth and the fourteenth year ; the limits can not be more accurately determined. In the first the temperature is lower titan in adult age, iu the second it is higher. The differences of temperature in the first age of infancy, and the adult age, although very sensible and impor tant as regards the economy, are indices of a difference incomparably greater than their numerical indication might be taken to imply. In fact, if the manner of observing be altered, results of so extraordinary a character are come to as to surpass all expectation. To deve lope these the temperature of the newly born being must not be taken only when it is in contact with its mother. If, after having as certained the temperature of a puppy in this position, it be removed from the mother and kept isolated, the temperature will be found to fall rapidly; and this phenomenon takes place not only when the air is cold, but when it is mild. The phenomenon does not commence after a term; it is apparent from the moment the separation takes place, and is very sensi ble after the lapse of a few minutes. 'The fol lowing is the rate of cooling of a puppy twenty four hours old, the external temperature being 13° c. (about 55°, 5 F.), taken at intervals of ten minutes ; the series of course represents the successive losses of temperature in the course of the small intervals of time indicated: —temperature in commencing the observations 36°, 87 c.; the declensions in temperature at intervals of ten minutes successively, 63, 12, 29, in thirty.five minutes the tem perature declined fart tr 1°, 25; in thirty-five minutes more it fell ; in thirty minutes more 2°,50; in twenty-five minutes more 1°,25; in thirty minutes more so that in the course of four hours in all the tempe rature declined by the amount of 18°, 12 of the centigrade scale (about F.) I Not only had the temperature of the animal sunk by so large a quantity in so short a period of time, the external temperature being pleasant, but it actually could maintain its temperature at no higher a grade than 75 c. 5, F.) above that of the atmosphere. Experiments of the same kind performed on three other puppies of the same litter presented results in all respects analogous. The cooling may even go much further by protracting the period during which the young animals are kept apart from their parent. For instance, four puppies, twenty four hours old and of much smaller size than the subjects of the former experiments, after having sunk 16° c. in four hours and thirty minutes, lost six degrees more of temperature in the succeeding eight hours and thirty mi nutes, the air remaining all the while at 13° c. (55°, 5 F.). They consequently lost twenty two degrees centigrade in thirteen hours ; and, what is very remarkable, their final temperature was but one degree above that of the surround ing air. Kittens and rabbits of the same age exhibited similar phenomena, if possible in a more striking degree. Some kittens were ob served to cool twenty degrees centigrade within the short interval of three hours and a half, and some young rabbits suffered the same de pression of temperature in two hours and ten minutes, the air being at the time at 14° c. (57°, 5 F.). These phenomena are unques tionably among the most remarkable we wit ness in warm-blooded animals. For here we have species of different genera of the Carni vora and Rodentia, which at two periods of their existence present the extremes in the pro ducunn of heat. They may be said to be, to all intents and purposes, cold-blooded ani mals, with reference to temperature, during the earliest period of life; they are only truly warm blooded animals in a later stage of their existence. The same phenomena undoubtedly present themselves in many other species ; but it would not be reasonable to suppose that they were exhibited by all.