67 
1907-8.] The Body-Temperature of Fishes, etc. 
blooded and cold-blooded. Homoiothermal animals were supposed to have 
a constant temperature in all atmospheres, and poikilothermal animals an 
inconstant or variable temperature. 
But objections might also be raised to this classification, for in no 
animal is the temperature absolutely constant, — there are diurnal and other 
variations, amounting in some instances to 3 or 4 degrees Centigrade, 
within the limits of health; and again, hibernating animals, such as the 
marmot and hedgehog, are homoiothermal in the summer and poikilothermal 
during their winter sleep, while all so-called homoiothermal animals are 
poikilothermal in the new-born condition. 
Strictly speaking, therefore, it would appear that no absolute distinction 
can be drawn between warm-blooded or homoiothermal animals on the 
one hand, and cold-blooded or poikilothermal animals on the other. 
Sutherland,* Macleay ]■ and Martin J have shown that the monotremes and 
marsupials form intermediate groups which can neither be described as 
homoiothermal nor poikilothermal; and in a paper on the physiological 
evolution of the warm-blooded animal Vernon § has endeavoured to 
prove that even the lowest of the so-called cold-blooded animals have 
some power of regulating their heat-production and body-temperature, 
and that as one ascends the series this power increases, until the most 
perfect regulation or homoiothermism is reached in man. In this relation 
might be mentioned the interesting fact that anatomical structure and 
zoological classification form no guide to physiological function. From 
the zoological point of view reptiles and birds are intimately related, 
both having sprung from a common ancestor, yet in the functional 
activity of their tissues and in the intensity of their metabolic processes 
the one is far removed from the other, reptiles being cold-blooded, and 
birds having a body-temperature some degrees above that of the higher 
mammals. 
While much work has been done on the production of heat and the 
regulation of temperature in warm-blooded or homoiothermal animals 
from the time of Black and Lavoisier onwards, this field has been left to 
a large extent unexplored in the case of poikilothermal animals, and 
it was with the view of adding something to our present knowledge 
of this subject that the observations recorded in the following pages 
were made. 
* Sutherland, Roy. Soc. Victoria Proc ., 1896, vol. ix., new series. 
t Macleay, Linn. Soc. N.S. Wales Proc., vol. ix,, 1st series, p. 1204. 
\ Martin, Phil. Trans. Roy. Soc., B., vol. cxcv., pp. 1-37. 
§ Vernon, Science Progress, vol. vii., 1898, p. 378. 
