
664 REVIEWS. 
closely allied or identical species. The fact that we are now to look for 
zones of life in which the genera are representative of former geological 
ages from the Cretaceous upwards, according to their depth, if true, is 
not less interesting than Professor Forbts’ original discovery of the 
bathymetrical distribut tion of marine forms. The investigators of the two 
faune h i i 
, X 
ence which could not be accounted for if there had been any very wide 
channel communicating between the two oceans since the existing spe- 
cies came into being. 
. Duncan, from his investigations in his article ** On the Fossil Corals 
of the West Indian Islands,"* is disposed to admit the connection of the 
genera resembling the present Indo-Pacific forms predominated in the 
Tertiary formation over those which are allied to genera now existing in 
the Caribbean seas, and Mr. J. C. Morse,t who has examined the fossil 
shells of San Domingo, confirms this view so far as to admit that Tertiary 
species like those now living in the Pacific are found in the rocks of that 
island. 
It would appear, therefore, from these conclusions, and those reached 
by Mr. Agassiz and Professor Verrill, that the connection of the faune 
must have been much more general in former geological periods, and that 
Indo-Pacific species actually did at one time cross their present bounda- 
ries and encroach upon the Atlantic, although subsequently driven back 
to their original limits 
The absolute identity of existing species of fishes common to both 
shores of Central America, and the similar physical conditions under 
which they exist, as pointed out by Dr. Günther, t forms another element 
in this curious pro 
If the Mire opuieatal hypothesis hes adopted, how shall we account for 
c 
-of physical causes or through natural selection, why did others, tmd 
allied to them anatomically, remain unchanged? Why do we not hav 
the frosh-water lakes of Managua a nd] Nicaragua some forms such as yo 
; shes found in the fresh-water lakes of Sweden, which 
were formed 4 the rise of the lands now dividing the Baltic from the 
Arctic 


— Why did "id Pacific fauna retreat after the Tertiary period, leaving, a8 
* Quarterly Journal Geological Society of London, xix, 1863. PM 
 T'Trans. Zool. Soc., London. VI, p. 397, 1868. 


