DEVELOPMENT OF HEAT. 



581 



velopment of the embryo of oviparous animals is entirely dependent upon the 

 amount of external warmth supplied to it ; and there are" many kinds of Birds, 

 which, at the time they issue from the egg, are so deficient in the power of 

 generating heat, that the temperature rapidly falls, when they are removed 

 from the nest and placed in a cold atmosphere. It was shown by collateral 

 experiments, that the loss of heat was not to be attributed to the absence of 

 feathers, nor to the extent of surface exposed in comparison with the bulk of 

 the body; and that nothing but an absolute deficiency in the power of gene- 

 rating it would account for the fall of temperature. This is quite conformable 

 to facts well ascertained in regard to Mammalia. The foetus, during intra- 

 . uterine life, has little power of keeping up its own temperature ; and in many 

 cases it is much dependent on external warmth, for some time after birth. 

 The degree of this dependence, however, differs greatly in the various species 

 of Mammalia, as among Birds ; being less, in proportion as the general deve- 

 lopment is advanced. Thus, young Guinea-pigs, which can run about and 

 pick up food for themselves almost as soon as they are born, are from the first 

 independent of parental warmth; whilst, on the other hand, the young of 

 Dogs, Cats, Rabbits, &c., which are born blind, and which do not, for a fort- 

 night or more, acquire the same development with the preceding, rapidly lose 

 their heat when withdrawn from contact with the body of the mother. In the 

 Human species it is well known, that external warmth is necessary for the 

 Infant ; but the fact is too often neglected (under the erroneous idea of hard- 

 ening the constitution) during the early years of childhood. It is to be care- 

 fully remembered, that the development of Man is slower than that of any 

 other animal ; and that his calorifying power is closely connected with his gene- 

 ral bodily vigour. In the case of children born very prematurely, the greatest 

 attention must be given to the sustenance of the heat of the body ( 756); and 

 though the infant becomes more independent of it as development advances, it 

 is many years before the standard can be maintained without assistance, 

 throughout the ordinary vicissitudes of external temperature. The calorify- 

 ing power, which is fully possessed by adults, decreases again in advanced 

 age. Old people complain that their " blood is chill;" and they suffer greatly 

 from exposure to cold, the temperature of their whole body being lowered by 

 it. These facts have a very interesting connection with the results of statisti- 

 cal inquiries, as to the average number of deaths at different seasons, recorded 

 by M. Q,uetelet.* 



We see from this table that, during the first month of infant life, the external 

 temperature has a very marked influence ; for the average mortality during 



* Essai de Physique Sociale, torn, i., p. 197. 

 49* 



