106 
MEMOIR OF ARISTOTLE. 
circulation of the blood, a9 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 difficulty 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 (si£os), and genus (ysnoj) ; 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 
