80 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



tion to various climates. Where the usual temperature of the atmo- 

 sphere is but little below the normal standard of the body, a small 

 amount of the internal calorifying power is required ; and accordingly 

 we find that animals which naturally inhabit the torrid zone, cannot be 

 kept alive elsewhere, except, like the Plants of the same regions, by ex- 

 ternal heat. On the other hand, the animals of the colder temperate 

 and frigid climes are endowed with a much greater internal calorifying 

 power ; and their covering is adapted to keep in the heat which they 

 generate. Such animals (the Polar Bear for example) cannot be kept 

 in health, in the summer of our own country, unless means are taken 

 for their refrigeration. The constitution of Man seems to acquire, by 

 habitation to a particular set of conditions through successive genera- 

 tions, an adaptation to differences of climate, of which that of few other 

 animals is susceptible ; and thus we find different races of human beings 

 inhabiting countries, which are subject to the extremes of heat and cold. 

 The Hindoo or the Negro, suddenly transported to Labrador or Siberia 

 during the depth of winter, would probably sink in the course of a few 

 days, from want of power to generate within his body a sufficient amount 

 of heat, to resist the depressing influence of the external cold ; whilst 

 on the other hand, the Esquimaux, suddenly conveyed to the hottest 

 parts of India or Africa, would speedily become the subject of disease, 

 which would probably terminate his life in a short time. It is in the 

 inhabitant of temperate climates, who is naturally exposed during the 

 seasonal changes of his year, to a wide range of external temperature, 

 that we find the greatest power of sustaining the extremes of either cold 

 or heat ; and yet, even in such, the continued exposure to either extreme, 

 during a long series of years, will so much influence the heat-producing 

 power, that the constitution does not adapt itself readily to a change of 

 conditions. 



114. We see, then, that the variations observable between different 

 races in this respect, are only exaggerations (so to speak) of the alter- 

 nations which an individual may undergo in the course of a few years ; 

 and it is easy to understand how such an adaptation may take place to 

 an increased extent in successive generations ; this being the regular 

 law, not merely in regard to Man, but in regard to other animals placed 

 under new conditions, to which they have a certain, but limited, power 

 of adapting themselves. Thus we find that a European, who has lived 

 for several years in the East or West Indies, suffers considerably from 

 the cold, when he first returns to winter in his native country ; his con- 

 stitution having, for a time, lost some of its power of generating heat. 

 After a few years' residence, however, this power is commonly recovered 

 to its original extent, unless the age of the individual be too far ad- 

 vanced ; but his Children, if they have been not only born, but brought 

 up, in the hotter climate, experience much greater difficulty in adapting 

 themselves to the colder one. 



115. The conditions on which the power of maintaining the heat of 

 the body, in despite of external cold, is dependent, will become the sub- 

 ject of inquiry hereafter (CHAP. x). It is sufficient here to state, that 

 this power is the result of numerous Chemical changes going on within 

 the body ; and especially of a process analogous to combustion, in which 



