106 MEMOIR OF ARISTOTLE. 



circulation of the blood, as well as the arrangement 

 of its nervous system, being essentially the same as 

 in viviparous quadrupeds. 



With reference to animal life generally, he notices 

 the gradual advances made by Nature from the state 

 of inanimate matter to that of living beings, whence 

 arose the difficult^ he felt in ascertaining the common 

 boundary of the two divisions. In the scale of ma- 

 terial existence, he observes that plants immediately 

 succeed to lifeless forms of matter ; holding, as it 

 were, a middle rank between animals and all other 

 organic bodies. His notion that inanimate sub- 

 stances graduate into life, as reptiles are alleged to 

 have sprung from the mud of the Nile, is erroneous; 

 but the difficulty which he felt in defining the ex- 

 act limit between animal and vegetable organization, 

 still exists, and is admitted by physiologists, after 

 the lapse of more than 2000 years. The only for- 

 mal terms of classification employed by Aristotle 

 are species (ido<;\ and genus (yzvo$) ; of the first of 

 which he gives a remarkably precise definition as 

 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 

 corresponding part in any other. His application of 

 the term genus is more vague, and sometimes ex- 

 tends to what we now denominate by tribe, family, 

 order, or even class. He was quite aware of the 

 necessary connexion between the blood and the life 

 of an animal : and he uses the colour of that fluid 



