DEVELOPMENT OF HEAT REGULATION. 865 
as do normal pigeons. The rapid rise in temperature which occurs 
when ;i hibernating marmot awakes, is not prevented by removal of the 
cerebral hemispheres. 
An impartial examination of the above evidence leads to the verdict 
that the existence of the so-called " heat centres " in the brain has not 
been proved. In the first place, the results, even in the hands of the 
same experimenter, are inconsistent ; some observers obtain exactly 
opposite effects from apparently similar lesions. Further, in many 
cases rabbits have been used for these experiments, and it is notorious 
that their temperature is liable to considerable variations during 
operative procedures. Even if the existence of these centres be 
granted, even if it be allowed that after puncture there is an increase in 
metabolism and in the production of heat, it by no means follows that 
the centres are special centres for the regulation of temperature, and 
give off " thermic nerves." 
It seems more probable that the mechanism of heat regulation 
has the same cerebral representation as the voluntary muscles. In 
the lower warm-blooded animals the representation of these in the 
cerebral cortex is not well developed, and it has likewise been shown 
that the removal of the cortex in them has little or no effect upon 
the regulation of temperature. 
The Development of the Power of Maintaining a Constant 
Temperature. 
In the cold-blooded animals there are traces of the power of maintaining 
a constant temperature, as shown by the high temperature which a female 
python is able to maintain for many weeks when she is incubating her eggs. 
This instance is the more remarkable because during that time the python 
takes no food or exercise. Further instances have already been mentioned in 
the case of bees, and some species of fish. 
It is possible to trace in the warm-blooded animals the gradual develop- 
ment of this power of regulation. Thus, during the development of a chick 
there is first a stage in which the embryo responds to changes of temperature 
in a similar manner to that of a cold-blooded animal ; then a stage of transition 
in which there is a regulation for moderate changes of temperature ; and 
finally, when a chick is hatched, the power of regulation resembles that of 
a warm-blooded animal. 1 In 1824, Edwards 2 pointed out that young 
mammals and birds may be divided into two classes, the warm-blooded 
and the cold-blooded, according as they are, or are not, able to maintain their 
temperature when removed from the warmth of the parents. The difference 
lies in the relative development of the two classes — active young animals 
covered with fur or feathers, as in the case of the guinea-pig and chick, belong 
to the former class ; while young animals born naked, blind, and helpless, 
belong to the cold-blooded group. The inability to maintain a constant 
temperature is due to diminished production of heat on exposure, and only 
secondarily to excessive loss of heat. It has recently been shown that the 
chick and guinea-pig can at birth regulate their production of heat, that young 
cold-blooded mammals and birds are able to regulate only f or moderate changes 
of external temperature ; for, when exposed to cold, their temperature and 
1 Pembrey, Gordon, and Warren, Journ. Physiol., Cambridge and London, 1894-95, 
vol. xvii. p. 331. 
2 " De linfluence des agens physiques sur la vie," 1824. 
VOL. I.— 55 
