INTRODUCTION. 53 



the less he acquires by any given addition to liis 

 stock ; a fact which, to a well informed man, be- 

 comes, like seed so^vn upon good ground, a tree 

 bearing fruit, and this always abundant, m precise 

 proportion to the accuracy and extent of his previous 

 iaformation ; while it is in the hands of an ignorant 

 man, a barren and a useless thing. It is tliis inca- 

 pacity for forming such associations which renders 

 the first steps to knowledge so difficult and weari- 

 some ; and it is this gradually increasing capacity 

 for forming such associations, which renders our 

 progress in a short time easy and light, and at 

 length almost intuitive, and in the highest degree 

 delightful and seducing. 



But are the different tribes of animals really 

 connected together by such intimate relations, as 

 that a knowledge of any one can always be made 

 subservient to the illustration of the rest ? At first 

 view, nothing can be more dissimilar in structure 

 than a quadruped and a fish. The former has its 

 head more or less erected on a neck fixed at an 

 angle with its body — it has a capacious chest 

 behind the neck — and it stands supported by legs : 

 in the latter, the head and body are in a line with 

 each other — it has no neck nor chest, properly so 

 called — and it is without proper legs, using other 

 organs, termed fins, in their place. Again, the 

 quadruped breathes by lungs, — while, in fishes, 

 the influence of the air is imparted to the blood and 

 system by means of gills; and in the former the 

 heart is double, — while in the latter it is single. 



