GENERATION OF ANIMALS 



Additional Note for II. 741 b 2. III. 762 b 24 fF. 



The first modern work on the breeding migration of the Thn 

 European eel {AnguUla milgaris) is that of Grassi " and Common 

 Calandruccio, who, following some previous work on ^°'- 

 the reproductive organs, made observations of eels in the 

 Mediterranean, and showed that Leptocephalus, already 

 known and described as a different animal, was the larval 

 form of the eel. The whole subject has been fully worked 

 out by Schmidt * in recent years. The facts are these. 

 During the time when eels live in fresh water, their repro- 

 ductive organs do not reach maturity, as Aristotle pointed 

 out ; but after a number of years, which may vary from five 

 to twenty, the body takes on a metallic sheen (" silver eels ") 

 and the fish set out on their migration to their breeding-places 

 in the deep waters between the West Indies and Bermudas. 

 The eggs float in the sea, and the larvae are carried by the 

 ocean currents eastwards across the Atlantic : upon arrival 

 at the Continental shelf two and a half years later they meta- 

 morphose into elvers, and these then move up into the 

 estuaries and rivers of Europe, sometimes passing over damp 

 grass to isolated pools. During the period of growth which 

 follows, they are yellowish and greenish in colour (" yellow 

 eels "). The old eels never return to fresh waters. The 

 story (mentioned by Aristotle) of the development of eels out 

 of horsehair worms was current until recent times. 



Additional Note for III. 757 a 2 ff. 



Aristotle discusses the hyena both here and at H.A. VI. The Hyena. 

 579 b 15 ff. 



An important piece of research on the spotted hyena 

 recently carried out in Tanganyika Territory by L. Harrison 

 Matthews ' has established that externally the female of 



« G. B. Grassi, Proc. Roy. Soc. LX (1897). 260-271. 



» J. Schmidt (of Copenhagen), The Breeding Places of the Eel, Phil. 

 Trans. Roy. Soc. (B) CCXI (1922). 179-208; see also id.. Nature, CXI 

 (1923). 51-54, CXIII (1924), 12; and W. Heape, Emiffration, Migration, 

 and Somadism, 1931. 



' Reproduction in the Spotted Hyena ((^rocuta crocuta), in Phil. 

 Trans. Roy. Soc. (B) CCXXX (1939), 1-78. 



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