THE METABOLISM OF THE BODY. 451 



to understand how by the law of habit and heredity each group 

 of animals has come to prefer and flourish best upon a certain 

 diet. But habit itself implies an original deviation some time, 

 in which is involved, again, plasticity of nature and power to 

 adapt as well as to organize. Without this, evolution of func- 

 tion is incomprehensible ; but with this principle, and the 

 tendency for what has once been done to be easier of rejwtition, 

 and, finally, to become organized, a flood of light is thrown 

 upon the subject of diet, digestion, and metabolism generally. 

 On these principles it is possible to understand those race differ- 

 ences, even individual differences, which as facts must be patent 

 to all observers. 



The principle of natural selection has clearly played a great 

 part in determining the diet of a species ; the surviving immi- 

 grants to a new district must be those that can adapt to the local 

 environment best, including the food which the region supplies. 

 The greater capability of resisting hunger and thirst in some 

 individuals of a species implies great differences in the meta- 

 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, hiber- 

 nation may be considered as a process of reversion, perhaps, for 

 the homoiothermer becomes very much a poikilothermer during 

 hibernation, the latter reverting to a condition existing in lower 

 forms, and not wholly unlike that of plants in winter. This 

 can be understood on the principle 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 sepa- 

 rated in form and function. If all higher groups bear a derivar 

 tive 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 homoiothsrmic (warm-blooded) condition 



