10 Proceedings of the Royal Physical Society. 



His " History of Animals " is the oldest and most celebrated 

 contribution to zoological science that has come down to us 

 from ancient times. The multitude of important and well- 

 ascertained facts he relates are reduced to a systematic ar- 

 rangement, and based on true physiological principles, that 

 subsequent discoveries, with all the aid of modern science, 

 have, with a few modifications, only tended to confirm. 



His " History of Animals " consists of nine books. The 

 opening chapter of the first book gives a general outline of 

 the animal kingdom, and offers suggestions for a natural 

 classification of animals, in accordance with their external 

 form and manner of life. He compares animals among them- 

 selves, and enumerates with surprising accuracy the agree- 

 ments, and differences, and analogies that prevail throughout 

 their form and structure. He shows himself acquainted with 

 the intimate relation that exists between the blood and the 

 life of an animal, and he makes use of the colour of the blood 

 for the primary division of the whole animal kingdom, which 

 he divides into animals possessing warm and red blood, and 

 those without blood -proper. This division is essentially 

 founded upon physiological principles, and the positive and 

 negative distinctions here indicated, under various forms and 

 modifications, still constitute the foundation of all our scien- 

 tific systems and classifications. 



The only constant and formal terms of classification em- 

 ployed by Aristotle are species {eidos) and genus {genus) ; and 

 he gives a remarkably clear and precise definition of species, 

 which he says is " an assemblage of individuals, in which not 

 only the whole form of any one resembles the whole form of 

 any other, but each part in any one resembles the correspond- 

 ing part in any other." His use of the term genus is more 

 vague, and sometimes extends to what is now understood by 

 tribe, family, order, or even class. With respect to animal 

 life in general, he notices in Book VIIL, that " nature passes 

 so gradually from inanimate matter to animated beings, that 

 from their continuity, their boundary, and the mean between 

 them is indistinct. The race of plants succeeds immediately 

 that of inanimate objects, and these differ from each other in 

 the proportion of life in which they participate ; for, compared 



