The phenomena being connected with the state of constitution, it may be expected to vary in different genera and families; and this, in fact, is what actually happens. A young guinea-pig, for instance, having a temperature of 38° c. (101°, 5 F.), will maintain this tem perature when the atmosphere is mild, although separated from its mother. It is the same with the goat. These instances are enough to give us a key to the external characters in relation with the different capacities to produce heat inherent in the young Mammalia. In the first place we observe a manifest relation with the state of energy of the nervous system : on the one hand we have the puppy, the kitten, the rabbit, which are born extremely weak ; on the other we have those animals that come into the world in a condition to walk, to eat, and, as it were, furnished forth to a certain extent with the means of providing for their wants. The question, however, is to discover some zoolo gical character in relation with these differ ences. If this were to be derived from the state of the organs of locomotion, of the faculty of walking, we should sometimes be led into error ; for man, at the period of his birth and long afterwards, is not in a condition to hold himself erect, and yet his temperature is main tained to within one or two degrees of that of his mother, if the external temperature be but mild. There is, however, one character that appears general ; this is the state of the eyes. Those species or Mammalia which in the earlier period of their existence do not maintain their temperature, that of the external atmosphere being mild or warm, but cool down to the standard rf the cold-blooded animals, are born with their eyes closed; whilst those which main tain their temperature, that of the external atmosphere being mild, are born with their eyes open ; and this, whether they can walk about like the guinea-pig, the kid, &c., or cannot do so, as is the case with the human infant in par ticular.
This general view of the state of energy of the nervous system in relation with the production of heat in early life, comes in aid, in a very remarkable manner, of the general principles which have been already deduced in regard to the calorific power. In going more deeply into the subject, the confirmation becomes more manifest and more complete. The state of the eyes affords a mere external and zoological in dication. It is but an indication of other deep modifications of the economy, which it is essential to determine more closely. Now in examining the state of the organs generally of puppies at the period of their birth, we observe a remarkable disposition of the sanguiferous system. The ductus arteriosus continues per vious and of large size. The consequence of this structure is that a free communication is established between the arterial and venous blood, by which they are mingled in large pro portion one with another. And here we have precisely the physiological character derived from the nature or quality of the blood which distinguishes the cold-blooded from the warm blooded Vertebrata (in the adult age under stood). This character is exactly the same in the other species of Alammalia which we have mentioned as losing temperature and attaining the standard of the cold-blooded tribes. On the other hand, in the guinea-pig, to take an individual instance, which from the first day of its extra-uterine existence maintains its tempera ture nearly on a level with that of its parent when the air is temperate, the ductus arteriosus is closed immediately after birth. The arterial remaining distinct from the venous blood, this creature is therefore born with the organization characteristic of warm-blooded animals, and presents phenomena having reference to calori fication of the same kind as adult warm-blooded animals.
This relation is preserved in the young Marnmalia in every modification in a peen harly interesting manner. The young Main Inaba which are born with the e3es closed, at first present the phenomena of refrigeration nearly in the same degree during the two or three first days of their life; though they after wards exhibit differences of great extent in this respect. Thus a young rabbit two days old had cooled down to 14° prom 23° c. (to 58° from F.) in the course of three hours fitly minutes, the air being at the time temperate; another three days old took seven hours twenty-five minutes to cool through a range of 18 c., when the process of refrigeration ceased. A third, of the age of five days only, lost 5° e. in temperature
in the course of one hour fifty-five minutes, and maintained itself afterwards at this tem perature. During the following days, smaller and smaller depressions of temperature were observed, till the eleventh day after birth, when the power of sustaining the temperature a little below that of the adult female parent seemed to be acquired permanently. V. hen the modifi cations of internal structure are examined during this interval of time, we find that the ductus arteriosus has been contracting in the same proportion as the faculty of maintaining the temperature has been increasing, and that it is entirely closed at the epoch when the tem perature becomes stationary, the external tem perature being understood all the while as mild or pleasant. At the same period pre cisely too, the eyes are unsealed, a circum stance which confirms the exactness of the character derived from the state of this latter organ, as distinctive of the young of those 111amtnalia which are born as it were cold blooded animals, from those that come into the world with the distinguishing attribute of warm blooded animals.
Among the young of Birds we observe as marked differences in the calorific function as we have just acknowledged among Mammalia. Some lose heat rapidly when separated from the mother ; others maintain their temperature to within a little of that of their species. Spar rows, for instance, which have been hatched but a short while, present a temperature from 4° to 5° c. lower than that of their parents when still contained in the nest, where they contribute•to each other's warmth. But taken out of the nest and isolated, although the temperature be that of summer they begin to cool with extreme rapidity. A young sparrow a few days old lost as many as 12° c. in the short space of one hour seven minutes, the air at the time marking 22° e. (72° F.). The same thing happens with swallows, sparrow-hawks, &c. But the law is not universal; it does not hold in re ference to all the genera. There are several that have the power of sustaining their tempera ture in spring and summer at a degree but little below that of their parents. Birds, there fore, form two groups as regards the production of temperature, just as the INIammalia do. The first cool down to the standard of cold-blooded animals; the second preserve their warmth, when the air is mild or agreeable as it is in the spring and summer. But the zoological characters that distinguish them are not the same as among mammiferous animals. All birds are hatched or born with their eyes open. But there are other characters which coincide with the difference of temperature; and this consists in the absence or presence of feathers. The covering of those that are hatched so pro vided, consists in a kind of down, very close and very warm, so that we might imagine the differences observed in the liability to lose heat or in the capacity to engender it, belonged to the coat. This has undoubtedly some influence, but analogy even will not suffer us to ascribe the chid effect to this cause. In the Mammalia which are born with their eyes closed, the refrigeration takes place to the same extent whether they are born with a fur-coat, as the kitten, the puppy, &c., or come into the world naked like the rabbit; the cooling is only more rapid in the latter than in the former. What further proves, and directly proves, that the refrigeration is not entirely due to the dif ference in the external condition as regards covering, although this of course must go for something, is that when the want of natural covering is artificially supplied, the cooling does not go on the less certainly oil this account, and to the same ultimate extent ; it only takes place somewhat more slowly. The counter proof is attended with the same result. An adult sparrow which has had all its feathers clipped off does not at first suffer loss of tem perature to the extent of more than a degree, and by-and-by recovers even this; whilst a young of the same species, though fur nished with some feathers, cools rapidly and to a great extent, as we have already seen. Birds are therefore divided into two groups as regards the production of heat. Time one comprises those that nre hatched with the skin naked, and which cool in a temperate air in the same manner as cold-blooded animals ; the other embraces those that are produced with a (lawny covering, and maintain their tcrnperatare at a considerable elevation in the ordinary heat of spring and sustainer.