8 GENERAL PRINCIPLES OF ZOOLOGY. 



ization find equal consideration. How far Aristotle, not- 

 withstanding many errors, attained to a correct knowledge 

 of the structure and embryology of animals, gains strong- 

 est proof from the fact that many of his discoveries have 

 obtained confirmation only in the present century. Thus 

 it was known to Aristotle, though only lately rediscovered 

 by Johannes Muller, that many sharks are not only vivip- 

 arous, but that also in their case the embryo becomes fixed 

 to the maternal uterus and there is formed a contrivance 

 for nutrition resembling the mammaliam and even the 

 human placenta; he knew the difference between male and 

 female cephalopods, and that the young cuttlefish has a 

 preoral yolk-sac. 



The position which Aristotle took in reference to the 

 systemization of animals is of great interest ; he mentions 

 in his writings the very considerable number of about 

 five hundred species. Since he does not mention the very 

 well known forms, like the badger, dragon-fly, etc., we can 

 with certainty assume that he knew many more, but that 

 he did not regard it necessary to give a catalogue of all 

 the forms known to him, and that he mentioned them 

 only if it was of consequence to refer to certain physio- 

 logical or morphological conditions found in them. 



This abeyance of systematic interest also finds expres- 

 sion in the fact that the great philosopher is satisfied with 

 two systematic categories, with eidos, species or kind, and 

 yeros or group. His eight yevij f^eyicrra would about 

 correspond with the Classes of modern Zoology ; they 

 have been the starting-point for all later attempts at 

 classification, and may therefore be here enumerated. 



1. Mammals (CCOOTOKOVVTCK ev avrois}. 



2. Birds (opvides). 



3. Oviparous quadrupeds (rer pot-nod a 



4. Fishes 



5. Mollusks 



6. Crustaceans 



7. Insects (evroj^a). 



8. Animals with shells 



