12 INTRODUCTION. 
the individual is possessed of their rationale, will he he 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 phenomene 
which they comprehend. very physiologist is now satisfied 
that the life or vital actions of no one species of animal car 
be correctly understood, unless compared with those of othe: 
tribes of different conformation. Hence, for the student o! 
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 o! 
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. No extremes of temperature in our atmo- 
sphere seem inconsistent with animal life. In the little pools 
