470 ANIMAL PHYSIOLOGY. 
bolic processes, though these are mostly unknown to us; and 
the same remark applies to heat and cold. 
It seems clear that hibernation is an acquired habit of the 
whole metabolism, with great changes in the functional condi- 
tion of the nervous system recurring periodically, and, in fact, 
dependent on these, by which certain large divisions, as the 
reptiles, amphibians, and certain mammals among vertebrates, 
are enabled to escape individual death and extinction as groups, 
We may suppose that, for example, among invertebrates, by a 
process of natural selection, those survived that could thus 
adapt themselves to the environment; while, among mammals, 
hibernation may be considered as a process of reversion, per- 
haps, for the homoiothermer becomes very much a poikilo- 
thermer during hibernation, the latter again reverting to a 
condition existing in lower forms, and not wholly unlike that 
of plants in winter. This can be understood on the princi- 
ple of the origin of higher from lower forms; otherwise it 
is difficult to understand why similar states of the metabolism 
should prevail in groups widely separated in form and func- 
tion. If all higher groups bear a derivative relation to the 
lower, what is common in their nature, as we usually find 
them, as well as the peculiar resemblances of the metabolism 
of higher to lower forms in sleep, hibernation, etc., can be 
understood in the light of physiological reversion. 
The origin of a homoiothermic (warm-blooded) condition 
itself is to be sought for in the principle of natural selection. 
It was open to certain organisms, we may assume, either to 
adapt to a temperature much below that of their blood, or to 
hibernate; failing to make either adaptation would result in 
death; and gradually, no doubt, involving the death of num- 
berless individuals or species, the resisting power attained the 
marvelous degree that we are constantly witnessing in all 
homoiothermers. 
The daily variations of the bodily temperature in homoio- 
thermers is a beautiful example of the law of rhythm evident 
in the metabolism. Hibernation is another such. While these 
are clear cases, it is without doubt true that, did we but know 
more of the subject, a host .of examples of the operation of this 
law might be instanced. 
We can but touch on these subjects enough to show that 
they deserve an attention not as yet bestowed on them; and to 
the thoughtful it will be evident that their influence on prac- 
tical life might be made very great were they but rightly ap- 
