238 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
South America, and while I do not wish to correlate all these formations, 
it seems very probable that they represent localities of the same general 
formation, representing the earlier half of the Tertiary period. 
American civil engineers familiar with the geography of the Magda- 
lena and the Chagres have assured me that there are extensive deposits 
of lignitic coals in Northern Colombia which are very similar in occur- 
rence and apparent age to those of the Isthmus. 
PART V. 
The Union of the Continents, and the Problems of 
the Straits. 
Let us now briefly compare the phenomena encountered along the 
continental cross sections and such fragmentary knowledge of the Isth- 
mian region as can be obtained from the writings of others with, what i8 
known of the geology of the whole Central American region, to ascertain 
what light can be obtained upon questions of the former existence of 
oceanic passages or land union between the two great continents of the 
western hemisphere. 
Many naturalists in studying the Pacific and Atlantic faunee, con- 
fronted by problems of geographic distribution, have established hypothe- 
ses involving former marine connections across the Isthmus of Panama. 
Such conclusions can be found in nearly every paper on the geographical 
distribution of marine animals. In many cases the reasons for such con- 
clusion, made by men prominent in their lines of research, have a sub- 
stantial foundation. In other instances similar hypotheses have largely 
been influenced by the narrow and elongate shape of the Isthmus. A 
glance at a map of Tropical America showing the narrow thread-like 
Isthmian band connecting the two continents always creates an impres- 
sion that the seas should have joined there, just as one is inclined to 
restore old land connections between the Antilles and the mainland, or 
to reconstruct Antillean continents. 
In the absence of geologic data it is easy to suppose that the Isthmus 
is a new made barrier, and the idea that it may be the remnant of an 
older decaying land does not at first, suggest, itself. The presence of a 
few common or similar species among the many dissimilar forms of the 
oceanie waters on either side of the barrier leads many to a belief in 
the recent existence of these passages across the supposedly new made 
Isthmian barrier. 
