4 EAKLY 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 Zoology was but little ad- 

 vanced by their efforts. 



We must come down to the last century, to 

 Linnaeus, 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 ; Linnaeus 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 



