262 Original Articles. [ April, 
quite comparable to that of warm-blooded animals; and this is effected 
by the retrograde metamorphosis of certain organic constituents of the 
food, of which we find the expression in the exhalation of carbonic 
acid and water. Thus the food of Animals becomes an internal 
source of heat, which may render them independent of external 
temperature.—Further, a like retrograde metamorphosis of certain 
constituents of the food is the source of that sensori-motor power which 
is the peculiar characteristic of the Animal organism ; for on the one 
hand the demand for food, on the other the amount of metamorphosis 
indicated by the quantity of carbonic acid exhaled, bear a very close 
relation to the quantity of that power which is put forth. This 
relation is peculiarly manifest in Insects, since their conditions of 
activity and repose present a greater contrast in their respective rates 
of metamorphosis, than do those of any other animals.— Of the exercise 
of generative force we have no similar measure ; but that it is only a 
special modification of ordinary vital activity appears from this 
circumstance, that the life of those Insects which ordinarily die very 
soon after sexual congress and the deposition of the ova, may be con- 
siderably prolonged if the sexes be kept apart so that congress cannot 
take place. Moreover, it has been shown by recent inquiries into the 
Agamic reproduction of Insects and other animals, that the process of 
Generation differs far less from those Reproductive acts which must 
be referred to the category of the ordinary Nutritive processes, than 
had been previously supposed. 
Thus, then, we find that in the Animal organism the demand for 
food has reference not merely to its use asa material for the con- 
struction of the fabric; food serves also as a generator of force; and 
this force may be of various kinds,—Heat and Motor-power being the 
principal but by no means the only modes under which it manifests 
itself. We shall now inquire what there is peculiar in the sources of 
the Vital Force which animates the organisms of the higher animals at 
different stages of Life. 
That the developmental force which occasions the evolution of the 
germ in the higher Vertebrata is really supplied by the Heat to which 
the ovum is subjected, may be regarded as a fact established beyond 
all question. In Frogs and other Amphibia, which have no special 
means of imparting a high temperature to their eggs, the rate 
of development (which in the early stages can be readily deter- 
mined with great exactness) is entirely governed by the degree of 
warmth to which the ovum is subjected. But in Serpents there is a 
peculiar provision for supplying heat; the female performing a kind 
of incubation upon her eggs, and generating in her own body a tem- 
perature much above that of the surrounding air.* In Birds, the 
developmental process can only be maintained by the steady appli- 
cation of external warmth, and this to a degree much higher than that 
* In the Viper the eggs are usually retained within the oviduct until they are 
hatched. In the Python, which recently went through the process of incubation 
in the Zoological Gardens, the eggs were imbedded in the coils of the body ; the 
temperature to which they were subjected (as ascertained by a thermometer placed 
in the midst of them) averaging 90° F., whilst that of the cage averaged 60° F. 
