XXVill INTRODUCTION. 
a cutting back of the western rivers has resulted in their capture, with the result 
that the eastern rivers with which they were formerly connected have diminished in 
size and fail to reach the main stream. 
The Rio Grande Province differs from the Mississippi Province principally in the 
paucity of types. Not one of the endemic nearctic families is present ; the Esocide are 
absent, and the Percide and Centrarchide are represented by a few species only. A 
considerable proportion of the true fresh-water fishes belong to the Cyprinide, whilst 
the presence of Cichlide and Characinide is a positive feature which distinguishes this 
area from other parts of the Nearctic Region. 
The Rio Lerma System (excluding the Rio Grande de Santiago below the falls, and 
properly including, as Dr. Meek has shown, the Rio San Juan, a tributary of the Panuco, 
as well as the isolated lakes in the States of Michoacan and Mexico) has so peculiar 
a fish-fauna that it may be regarded as a separate sub-region of the Nearctic Region. 
The viviparous Cyprinodontide of the sub-family Characondontine are characteristic 
of and nearly peculiar to this, the Lerma, Sub-region, in which the Atherinid genus 
Chirostoma is represented by a number of species which show a remarkable diversity. 
Both these groups are probably derived from marine ancestors which entered the river 
at a remote epoch; none of the marine types (Gobiide, Mugilide, &c.) which are 
found at the present day in neighbouring rivers, such as the Balsas and Panuco, have 
been able to make their way into the Lerma System, from which neotropical fishes are 
also absent. 
The Cyprinide of this sub-region differ considerably from those of the Rio Grande, 
as five of the seven genera are endemic. With the exception of a Cat-fish (Amiurus) 
and a Lamprey (Lampetra), all the fishes of the Lerma Sub-region belong to the three 
groups already mentioned. 
Below, in comparing some of the shore-fishes of the Pacific and Atlantic coasts of 
Central America, it is shown that in many cases D. S. Jordan’s generalization—to the 
effect that a form occurring in a certain area has as its nearest relative a form 
inhabiting a neighbouring area separated by some sort of barrier from the first—holds 
good. In such cases isolation appears to have been a factor in determining specific 
differentiation. 
Jordan’s generalization also holds good for many groups of fresh-water fishes, but 
for many others it does not. As examples of the latter we may instance the Cichlid 
fauna of Lakes Managua and Nicaragua and the greater part of the fish-fauna of the 
Lerma System. 
