POST-SOCRATIC SCIENCE AT ATHENS 



physiologist calls the homologies of parts of the 

 organism. 



Aristotle divides his so-called blood -bearing animals 

 into five classes: (i) Four-footed animals that bring 

 forth their young alive; (2) birds; (3) egg-laying four- 

 footed animals (including what modern naturalists 

 call reptiles and amphibians) ; (4) whales and their 

 allies; (5) fishes. This classification, as will be ob- 

 served, is not so very far afield from the modern di- 

 visions into mammals, birds, reptiles, amphibians, and 

 fishes. That Aristotle should have recognized the 

 fundamental distinction between fishes and the fish- 

 like whales, dolphins, and porpoises proves the far 

 from superficial character of his studies. Aristotle 

 knew that these animals breathe by means of lungs and 

 that they produce living young. He recognized, there- 

 fore, their affinity with his first class of animals, even 

 if he did not,, like the modern naturalist, consider these 

 affinities close enough to justify bringing the two types 

 together into a single class. 



The bloodless animals were also divided by Aris- 

 totle into five classes namely: (i) Cephalopoda (the 

 octopus, cuttle-fish, etc.) ; (2) weak-shelled animals 

 (crabs, etc.); (3) insects and their allies (including 

 various forms, such as spiders and centipedes, which 

 the modern classifier prefers to place by themselves) ; 



(4) hard-shelled animals (clams, oysters, snails, etc.) ; 



(5) a conglomerate group of marine forms, including 

 star-fish, sea-urchins, and various anomalous forms 

 that were regarded as linking the animal to the vege- 

 table worlds. This classification of the lower forms of 

 animal life continued in vogue until Cuvier substituted 



