VOYAGE FROM NEW YORK TO RIO DE JANEIRO. 33 



Mobile no less than six new species in the course of an af- 

 ternoon's ramble. These fishes are almost all viviparous, or 

 at least lay their eggs in a very advanced state of develop- 

 ment of the young. The sexes differ so greatly in appear- 

 ance that they have sometimes been described as distinct 

 species, nay, even as distinct genera.* We must be on our 

 guard against a similar mistake. Here again we have two 

 groups, the Cyprinoids and Cyprinodonts, so similar in 

 their structural features that the development of one out 

 of the other naturally suggests itself. But in South Amer- 

 ica there are no Cyprinoids at all, while the Cyprinodonts 

 abound ; in Europe, Asia, and North America on the con- 

 trary, the Cyprinoids are very numerous and the Cyprino 

 donts comparatively few." The Characines were next con- 

 sidered with reference to their affinities as well as their 

 geographical distribution ; and a few remarks were added 

 upon the smaller families known to have representatives in 

 the fresh waters of South America, such as the Erythri- 

 noids, the Gymnotines, &c. " I am often asked what is my 

 chief aim in this expedition to South America ? No doubt 

 in a general way it is to collect materials for future study. 

 But the conviction which draws me irresistibly, is that the 

 combination of animals on this continent, where the faunae 

 are so characteristic and so distinct from all others, will gi^e 

 me the means of showing that the transmutation theory is 

 wholly without foundation in facts." The lecture closed 

 with some account of the Salmonidae, found all over the 

 northern hemisphere, but represented in South America by 

 the Characines, distinct species of which may be looked for 

 in the separate water-basins of Brazil ; and also of several 



H 



other important families of South American fishes, espe- 



* Molinesia and Poecilia. 



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