PROGRESS OF SYSTEMATIC ZOOLOGY. 387 



and the animal which in Syria is called 

 heminus (mule) ; for these are called mules from 

 their resemblance only; not being mules, for they 

 breed of their own kind. Wherefore," he adds, that 

 is, because we do not possess recognized genera 

 and generic names of this kind, " we must take the 

 species separately, and study the nature of each." 



These passages afford us sufficient ground for 

 placing Aristotle at the head of those naturalists to 

 whom the first views of the necessity of a zoolo- 

 gical system are due. It was, however, very long 

 before any worthy successor appeared, for no ad- 

 ditional step was made till modern times. When 

 natural history again came to be studied in nature, 

 the business of classification, as we have seen, forced 

 itself upon men's attention, and was pursued with 

 interest in animals, as in plants. The steps of its 

 advance were similar in the two cases; by suc- 

 cessive naturalists, various systems of artificial marks 

 were selected with a view to precision and conve- 

 nience ; and these artificial systems assumed the 

 existence of certain natural groups, and of a natural 

 system to which they gradually tended. But there 

 was this difference between botany and zoology : 

 the reference to physiological principles, which, as 

 we have remarked, influenced the natural systems 

 of vegetables in a latent and obscure manner, bo- 

 tanists being guided by its light, but hardly aware 

 that they were so, affected the study of syste- 

 matic zoology more directly and evidently. For 



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