What we have said of animals is equally applicable to vegetables. To explain the pro gression of the temperature of cold-blooded animals, which we have expnsed above, regard must be had to the relation which connects the quantity of vapour formed with the degree of external temperature. Within moderate limits, which may be styled temperate, the vapour funned will be nearly as the degrees of tem perature of the air. But under higher tempera tures, evaporation will go on in a greater ratio than that of the external temperature. Thus when the air is cool or moderately warm, eva poration is trifling, and among the superior classes of cold-blooded animals heat enough is produced to maintain their temperature above that of the air. But when the air becomes warmer, as in the height of summer, evapora tion and the cold which results from it increase in a far greater ratio than the temperature of the body, so that the body remains at a tem perature inferior to that of the air, and this by so much the more as the external tempera ture rises higher. Twenty-five degrees is the limit at which this change commences in regard to cold-blooded animals. But it is obvious that a higher degree must be necessary to ob serve such phenomena in man and the warm blooded tribes, inasmuch as the heat from without is for a long time added to that pro duced internally, and which among the warm blooded tribes is so much greater in amount than it is among the cold-blooded.
Relations of the bulk of the body with animal heat.—If the temperature of the larger animals be compared with that of the smaller, it will be found that the former do not mark so high a degree as the latter. In the elephant and horse, for instance, no higher a temperature than 37°, 5 c. (100° F.) has been observed, whilst in the rat and squirrel temperatures of 38°, 8, and of 39°, 4 (102° and F.) have been noted. To prove that the difference is less owing to the order or species than to the simple size, we shall contrast several animals belonging to the same order, selecting the ruminants. The temperature of the air being the same, namely, 26° c. (79° F.), the tem perature of the ox was found to be 38°, 9 (102° F.), whilst that of a castrated he-goat was 5 F.), and that of the she-goat and sheep 40° (104° F.).* It is evident that smallness of size must in itself be one of the conditions unfavourable to height of temperature among animals, when this is merely viewed in relation with the am bient medium. As the external temperature is almost always lower than that of the bodies of animals, the ambient medium tends to lower their temperature ; and small bodies having a more extensive surface in reference to their mass than large bodies, small animals must have a greater tendency to lose heat than larger animals.
But, on the other hand, the circulation and respiratory motions generally increase in rapidity in proportion to the smallness of size ; and we have seen that acceleration of these motions had an influence in keeping up the temperature. With a small size of the body, consequently, we find associated a higher activity of function which tends to compensate the disadvantage resulting from inferior size in reference to tem perature. In fact it constantly happens that this higher activity more than compensates the cooling disposition from inferiority of size, and causes the balance to incline towards the side of higher temperature. It must be apparent, however, that there is no occasion for such a preponderance always existing in the case of small animals. And then we know that the motions of circulation and of respiration cannot be greatly accelerated without causing incon venience and even danger to health and life. It follows that the external temperature being liable to fall disproportionately low, small ani mals have not, under like disadvantageous cir cumstances, the same power as larger animals of supporting their temperature. The relations of size naturally lead us to consider those that depend on age.
Relations of age with animal heat.—The size of the body changes with the age. The same relations between bulk of body and de velopment of heat ought therefore to be ex hibited in youth as compared with adult age. In early life the greater rapidity of the motions of circulation and respiration, all things else being equal, ought to increase the heat. At the same time the constitution differs in other respects, and if these were unfavourable to the evolution of heat, it would be impossible to foresee the result of these two opposite ten dencies. Nevertheless it is probable, from what we have seen to happen in warm-blooded animals of different sizes, that there might occur a period in early life when the heat would be higher than in adult age. A confirmation of this inference may be found in comparing the different observations of Dr. Davy, who has given a table of the temperatures of fifteen chil dren from four to fourteen years, the mean age of the whole being nine years and nine months. The mean temperature of the bodies of these children was 38 , 31 F.). But the mean temperature of twenty-one adults was no higher than (l00° F.); a difference that seems the more worthy of being confided in from the temperature of the air having, at the time of the observations, been more favourable for the adults than for the children, this having, in re ference to the former, been 26° and 7 (79° and 80° F.), whilst when the latter were made the subjects of investigation, it was but and 26° 5 and F.).