It is of great importance that a precise idea be formed of this expression. Up to a very recent period in the investigation of animal heat, no one thought of comparing animals save with reference to the temperature of their bodies only : and when it was found that this was the same or different by so much, the ac count was closed, the comparison was pushed no farther, under the impression that every thing was included under this single ostensible character. Undoubtedly, it must be granted that, all else being alike, equality of tempera ture is an indication of equality in the capacity to produce heat. But animals in one set of circumstances may actually produce the same quantity of caloric, and not continue to do this the circumstances being changed. It is of consequence to distinguish the actual produc tion, from the power to produce under different conditions. The one is an act, the other a fa culty, a distinction of the highest importance in philosophical language in general, and espe cially in that of physiology. But animals of the same size, subjected to the same variations of external conditions, if they continue to ex hibit corresponding degrees of temperature, whether these are higher or lower, have evi dently the same faculty of producing heat. If,
on the contrary, they present degrees under the influence of precisely similar exter nal variations of circumstance, it is obvious that they must possess the faculty of producing heat in different degrees. Unless we be actu ally persuaded of the value of this expression, so simple in other respects, and so constantly held in view in all analogous circumstances, the study of the phenomena of animal heat would remain as it were barren, whilst the in vestigation of the diversities of constitution in relation with this faculty is fertile in interesting and useful applications.
We have seen how constitutions differed in this respect according to age in the earlier period of life and in the adult state. It is probable that there are other varieties depend ent on other causes; for example, differences of season, climate, &c. This point it will be our next business to examine.