1868. | Reviews. 489 
remainder returned down the river as far as Teffé, where they 
remained nearly a month, journeying back then to Manaos, which 
they made their head-quarters and general rendezvous. After the 
reassembling of the party, and after making some excursions on the 
river Ramos and the Rio Negro, their steps were retraced to Para, 
and thence to Rio. Remaining in the capital, and exploring its 
neighbourhood, for about three months, the expedition returned to 
New York after a sojourn of fifteen months in Brazil. 
The known results of this expedition are too numerous to 
mention even in a review, and many facts and conclusions of the 
highest importance still remain to be worked out and verified before 
any cautious naturalist would venture to publish them. The chief 
object of the expedition was to ascertain how the fresh-water fishes 
are distributed through the great river-systems of Brazil; and all 
the excursions and independent journeys were planned in reference 
to this idea. Professor Agassiz estimates the number of species of 
fresh-water fish collected by the members of the expedition at 
between 1800 and 2000, or nearly twice as many species as exist 
in the Mediterranean. The number, however, is not so astonishing 
as the distribution. From Tabatinga to Para the river differs 
neither in the temperature of its waters, in the vegetation of its 
banks, nor in the nature of its bed; yet under these circumstances 
completely distinct assemblages of fish are met with from distance 
to distance. When we consider that all the rivers of Europe united 
have not yielded more than 150 species of fresh-water fish, and that 
Professor Agassiz found in one small pond, covering about 400 or 
500 square yards, no less than 200 species, mostly peculiar to that 
spot, we are inclined to indulge in a little scepticism. We want to 
know how many genera and families are represented by these 200 
species, and what degree of affinity there is between the species 
themselves. Remembering Professor Agassiz’s extreme opinions in 
favour of the theory of special and direct creation of species, we 
cannot help regarding it as possible that his “ species” are not all 
entitled to that dignity. If they are, and are closely related, the 
explanation may be that the process of variation is stimulated in a 
vast degree by the tropical climate and physical features of the 
region. If both these suppositions are erroneous, we have here a 
Natural History marvel entirely without precedent ; for we presume 
that the majority of Mr. Lea’s species of Unionide, the only com- 
parable example we know of, would be regarded as varieties by 
most European conchologists. 
Another “sensation” discovery is that of the evidences of the 
Glacial Period in tropical America. Every geologist knows that 
the crystalline rocks of South America are extensively decomposed 
at the surface; but Professor Agassiz states that although the 
received explanation is true to a great extent, yet that a consider- 
