302 BIOLOGY: GENERAL AND MEDICAL 



Blooded Animals. Fishes 



Amphibia 

 Birds 



. Mammals 



Here the matter rested for centuries without important 

 addition or alteration, for religion displaced science and 

 philosophy, which were almost forgotten until the 

 Renaissance. 



The next important classification comes to us from 

 Linnaeus, the father of botany and a profound thinker 

 and reasoner. Not a versatile zoologist, but one to 

 whom zoology owes much in the introduction of the 

 binomial nomenclature and in the description of all the 

 common forms of animals, he gives us the following 

 very simple arrangement: 



I. Mammalia 

 II. Aves 



III. Amphibia 



IV. Pisces 

 V. Insects 



VI. Vermes 



Linnaeus thus departs from Aristotle by dispensing with 

 the two primary divisions of Bloodless and Blooded 

 animals. 



Aristotle based the primary groupings upon the pres- 

 ence or absence of (red) blood, but we find that Lin- 

 nseus abandoned this feature. This leads us to inquire 

 what are the legitimate characters upon which classifi- 

 cation can be based. 



Such characters are purely arbitrary and at the option 

 of the systematist by whom the classifying is done. 

 But in the arbitrary selection of the characters used for 

 the purpose one thing is essential, viz., that they be 

 constant. They need not bear any reference to struc- 

 tural or functional importance, but they must be invari- 

 able. Now, when we scrutinize Aristotle's employment 

 of the presence or absence of blood as a primary differ- 

 ential character, we find it to be inconstant. Blood was, 



