Cuap, ll. FORMATION OF THE AMAZONS DELTA. 55 
By this it is meant that the two regions have a very large number of 
forms peculiar to themselves, and which are supposed not to have been 
derived from other quarters during modern geological times. Each 
may be considered as a centre of distribution in the latest process of 
dissemination of species over the surface of tropical America. Para 
lies midway between the two centres, each of which has a nucleus of 
elevated table-land, whilst the intermediate river-valley forms a wide 
extent of low-lying country. It is, therefore, interesting to ascertain 
from which the latter received its population, or whether it contains so 
large a number of endemic species as would warrant the conclusion that 
it is itself an independent province. To assist in deciding such questions 
as these, we must compare closely the species found in the district 
with those of the other contiguous regions, and endeavour to ascertain 
whether they are identical, or only slightly modified, or whether they 
are highly peculiar. 
Von Martius, when he visited this part of Brazil forty years ago, 
coming from the south, was much struck with the dissimilarity of the 
animal and vegetable productions to those of other parts of Brazil. In 
fact, the Fauna of Para, and the lower part of the Amazons, has no 
close relationship with that of Brazil proper; but it has a very great 
affinity with that of the coast region of Guiana, from Cayenne to Demerara. 
If we may judge from the results afforded by the study of certain 
families of insects, no peculiar Brazilian forms are found in the Para 
district ; whilst more than one-half the total number are essentially 
Guiana species, being found nowhere else but in Guiana and Amazonia. 
Many of them, however, are modified from the Guiana type, and about 
one-seventh seem to be restricted to Para. ‘These endemic species are 
not highly peculiar, and they may be yet found over a great part of 
Northern Brazil, when the country is better explored. They do not 
warrant us in concluding that the district forms an independent province, 
although they show that its Fauna is not wholly derivative, and that the 
land is probably not entirely a new formation. From all these facts I 
think we must conclude that the Para district belongs to the Guiana 
province, and that, if it is newer land than Guiana, it must have 
received the great bulk of its animal population from that region. I 
am informed by Dr. Sclater that similar results are derivable from the 
comparison of the birds of these countries. 
The interesting problem, How has the Amazons Delta been formed ? 
receives light through this comparison of Faunas. Although the 
portion of Guiana in question is considerably nearer Para than are the 
middle and southern parts of Brazil, yet it is separated from it by two 
wide expanses of water, which must serve as a barrier to migration 
in many cases. On the contrary, the land of Brazil proper is quite 
continuous from Rio Janeiro and Bahia up to Parad; and there are 
no signs of a barrier ever having existed between these places within 
recent geological epochs. Some of the species common to Pard and 
Guiana are not found higher up the river where it is narrower, so they 
could not have passed round in that direction. The question here 
arises, Has the mouth of the Amazons always existed as a barrier to 
migration since the present species of the contiguous regions came into 
