4 EARLY PROGRESS 
however, that by the word blood he designated 
only the red fluid circulating in the higher ani- 
mals ; whereas a fluid akin to blood exists in all 
animals, variously colored in some, but colorless 
in a large number of others. 
After Aristotle, a long period elapsed without 
any addition to the information he left us. 
Rome and the Middle Ages gave us nothing, and 
even Pliny added hardly a fact to those that 
Aristotle recorded. And though the great nat- 
uralists of the sixteenth century gave a new 
impulse to this study, their investigations were 
chiefly directed towards a minute acquaintance 
with the animals they had an opportunity of 
observing, mingled with commentaries upon the 
ancients. Systematic Zodlogy was but little ad- 
vanced by their efforts. 
We must come down to the last century, to 
Linnzus, before we find the history taken up. 
where Aristotle had left it, and some of his sug- 
gestions carried out with new freshness and vigor. 
Aristotle had already distinguished, between gen- 
era and species; Linnzus took hold of this idea, 
and gave special names to other groups, of dif- 
ferent weight and value. Besides species and 
genera, he gives us orders and classes, — con- 
sidering classes the most comprehensive, then 
orders, then genera, then species. He did not, 
however, represent these groups as distinguished 
