ANIMAL HEAT. 



665 



liarly interesting manner. The young Mam- 

 malia which are born with the eyes closed, at 

 first present the phenomena of refrigeration 

 nearly in the same degree during this 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 from 23 c. (to' 58 from 

 74 F.) in the course of three hours fifty 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 5c. 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. \Vhen 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 

 Mammalia 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 c. (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 Mammalia 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 chief 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 on 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 bird of the same species, though fur- 

 nished with some feathers, cools rapidly and to 

 a great extent, as we have already seen. Birds 

 are there/ore divided into two groups as regards 

 the production of heat. The one comprises 

 those that are hatched with the skin naked, and 

 which cool in a temperate air in the same 

 manner a-s cold-blooded animals ; the other 

 embraces those that are produced with a 

 downy covering, and maintain their temperature 

 at a considerable elevation in the ordinary heat 

 of spring and summer. 



There is not a less remarkable contrast 

 between these two groups of birds in point 

 of calorific power, than between the two 

 groups of Mammalia already mentioned ; but 

 the zoological or external characters which dis- 

 tinguish them in the present instance are not of 

 the same kind. The state of the eyes does not 

 apply here, for all Birds are disclosed with their 

 eyes unsealed. They also all come into the 

 world with the ductus arteriosus closed or nearly 

 so, a circumstance which might have been 

 predicated, or inferred from analogy. Yet the 

 young of Birds in the power of producing heat 

 present diversities no less remarkable than are 

 observed among the young of the Mammalia. 

 The separation of the two kinds of blood con- 

 sequently is not the only condition which 

 influences the production of heat; but all that 

 modifies the blood on the one hand, and the 

 nervous system on the other, as we have had 

 occasion to observe in a previous part of this 

 paper. Now it happens that we have an op- 

 portunity of applying this principle in a very 

 particular manner in the instance of the two 



