ZOOLOGY. 



Plates 74-118. 



General Introduction. 



Zoology is a systematic exposition of animals according to their external 

 and internal structure, and the functions of their organs. The internal 

 structure cfcn be frequently inferred from the external characters ; we may, 

 for example, determine the aliment of an animal, and the structure of the 

 digestive organs, by examining the teeth. This is, however, not suflicient 

 in all cases, so that it becomes necessary also to examine the internal parts, 

 because the relation between animals depends upon the entire organization ; 

 and this being well ascertained, tlie functions of the various organs can 

 generally be determined without much difficulty. 



From the earliest period it was found necessary to group those animals 

 together which were observed to have certain natural characters in common. 

 AVe find, upon inquiry, that the endeavors to arrange animals systematically 

 have taken two principal directions, which have been named natural and 

 artificial classification. The former has in view the classification of animals 

 upon the greater or less perfection of the various organs, among which 

 those connected with the circulation and oxygenation of the blood, 

 locomotion, and digestion, hold a prominent place ; the latter depends upon 

 a character or habit arbitrarily chosen, and independent of others. A 

 character, however, which may be regarded as unessential by one observer, 

 will be considered as of the greatest importance by another. In the earlier 

 stages of science, when the number of known species was comparatively 

 small, artificial methods were popular, because they were considered easy 

 of acquisition : now, however, it is found that they are calculated to give 

 superficial ideas ; and that to present the condition of zoological science in 

 its true light, a more philosophical system must be made use of. 



Aristotle, whose great mind was master of many sciences, both moral and 

 physical, and whose works had an authority in Europe for many centuries, 

 second only to that of the sacred Scriptures, takes precedence, in j)oint of 

 time, as the first systematic zoological observer. Born at Stagira, in the 

 year 383 before Christ, he became the instructor of Alexander the Great, 

 who formed a large collection of animals to enable him to jjursue his investi- 

 gations. As a history of zoological systems forms no part of the plan of 

 this work, only a short outline of the most important periods will be 

 given. 



Akistotle divides animals into such as have blood, and such as are 



205 



