12 INTRODUCTION. 



the individual is possessed of their rationale, will he be likely 

 to shape his conduct in accordance with them. 



The general principles of Physiological science, again, will 

 be likely to be thoroughly apprehended, in proportion as they 

 are based on an extended recognition of the phenomena 

 which they comprehend. Every physiologist is now satisfied 

 that the life or vital actions of no one species of animal can 

 be correctly understood, unless compared with those of other 

 tribes of different conformation. Hence, for the student of 

 physiology to confine himself to the observation of what 

 takes place in Man alone, would be as absurd as for the astro- 

 nomer to restrict himself to the observation of a single planet, 

 or for the chemist to endeavour to determine the properties of 

 a metal by the study of those of that one only. There is not 

 a single species of animal, that does not present us with a set 

 of facts which we should never learn but by observing it ; 

 and many of the facts ascertained by the observation of the 

 simplest and most common animals, throw great light upon 

 the great object of all our inquiries, the Physiology of Man. 

 For though in him are combined, in a most wonderful and 

 unequalled manner, the various faculties which separately 

 exhibit themselves in various other animals, he is not the 

 most favourable subject for observing their action ; for the 

 obvious reason that his machinery (so to speak) is rendered 

 too complex, on account of the multitude of operations it has 

 to perform : so that we often have to look to the lowest and 

 simplest animals for the explanation of what is obscure in 

 man, their actions being less numerous, and the conditions 

 which they require being more easily ascertained. 



The diffusion of Animal life is only one degree less exten- 

 sive than that of vegetable existence. As animals cannot, 

 like plants, obtain their support directly from the elements 

 around, they cannot maintain life, where life of some kind 

 has not preceded them. But vegetation of the humblest 

 character is often sufficient to maintain animals of the highest 

 class. Thus the lichen that grows beneath the snows of 

 Lapland, is, for many months in the year, the only food of 

 the rein-deer ; and thus contributes to the support of human 

 races, which depend almost solely upon this useful animal for 

 their existence. JSTo extremes of temperature in our atmo- 

 sphere seem inconsistent with animal life. In the little pools 



