238 bulletin: museum of comparative zoology. 



South America, and while I do uot 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 is 

 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 faunse, 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. Iu 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 

 oceanic 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. 



