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 give 
me the means of showing that the transmutation theorj is 
wholly without foundation in facts." The lecture closed 
with some account of the Salmonidaa, 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 
other important families of South American fishes, espe- 
* Molinesia and Pcecilia. 
3 
