38 INTRODUCTION. 



The motions of flying and swimming, and the means 

 by which an animal can so alter its specific gravity or 

 weight, in proportion to its bulk, as to be able to ascend 

 and descend, and also to float in mediums of different 

 densities, are still more curious than those of progressive 

 motion along the earth. They are performed partly 

 by the muscular power of wings and fins, and partly 

 by the help of air-cells and air-vessels, which the ani- 

 mal can expand or compress at pleasure; but their 

 principles, as they involve a mechanical and pneumatic 

 action at the same time, are rather more difficult to 

 explain. By observing the habits, and examining the 

 structure of the animal, we may however obtain some 

 knowledge of them ; but in the most interesting parts 

 of the study, that of the instincts and dispositions of 

 the animal as a living creature, we can infer nothing 

 but that two animals, which are exactly alike in their 

 structure, will be of the same disposition ; and though 

 that be a very general rule, as established by experience, 

 it is not universal. 



Hence the only sure way to become naturalists, in 

 the most pleasing sense of the term, is to observe the 

 habits of the plants and animals that we see around us, 

 not so much with a view of finding out what is uncom- 

 mon, as of being well acquainted with that which is of 

 every day occurrence. Nor is this a task of difficulty, 

 or one of dull routine. Every change of elevation 

 or exposure, is accompanied by a variation both in 

 plants and in animals ; and every season and week, nay 

 almost every day, brings something new ; so that while 

 the book of nature is more accessible and more easily 

 read than the books of the library, it is at the same 



