January 28, 1916] 



SCIENCE 



123 



which has been studied by Macdonald. 

 There is very little direct information, as 

 yet, regarding the condition of Central 

 America and the Isthmus of Panama dur- 

 ing the Eocene, whatever rocks there may 

 be in the region of Eocene or earlier date 

 being buried under newer formations. It 

 is clear, however, that in the succeeding 

 Oligocene epoch the whole Caribbean re- 

 gion was extensively submerged; the 

 Greater Antilles were much reduced in 

 size and nearly the whole of Central Amer- 

 ica was under water, a broad sea sepa- 

 rating North and South America, though 

 doubtless with scattered islands. On the 

 Caribbean side of the Isthmus is a very 

 thick mass (estimated at 2,500 feet) of 

 Oligocene strata, the Gatun formation, 

 which is crowded with marine fossils. 

 The Culebra Hills, through which the great 

 cut has been made, are chiefly built up of 

 volcanic materials, lava streams, mud-flows, 

 tufEs, etc., in a very complicated arrange- 

 ment, and running through the cut may be 

 traced thin bands of a marine limestone, 

 which carry Oligocene fossils. The evi- 

 dence of submergence is thus complete, but 

 the date of upheaval can not be very defi- 

 nitely fixed. Except for a narrow strip of 

 Pleistocene on the Caribbean coast, no ma- 

 rine rocks later than the Oligocene have 

 been found on the Isthmus and it would 

 be natural to conclude that the elevation 

 came at the close of that epoch. Some al- 

 lowance must, however, be made for erosion 

 and it is quite possible that early Miocene 

 rocks were formed and have since been 

 swept away. The ground-sloth in the mid- 

 dle Miocene of Oregon is proof that the 

 connection was established at least as early 

 as that. 



In the Pliocene and perhaps early 

 Pleistocene the isthmian region was con- 

 siderably broader than it is now and it is 

 probable that the flat lands lying along the 



then existing coast afforded a compara- 

 tively easy highway of migration, the chief 

 obstacles to which were climatic rather 

 than topographical, but during some part 

 of the Pleistocene the Isthmus was again 

 depressed and narrowed even beyond its 

 present limits, as is shown by the fringe of 

 marine deposits along the Caribbean coast. 



Central America, like the West Indies, 

 belongs zoologically to South America and 

 forms a part of the Neotropical Region. 

 This fact is not altogether easy of explana- 

 tion. It may be that the Isthmus connected 

 South and Central America at a time when 

 the latter was still separated from the 

 northern continent. Were this the case, 

 the southern fauna would have had the ad- 

 vantage of possession, when the northern 

 invasion began. On the other hand, the 

 cause may be entirely climatic and that 

 this is the rightful conclusion is indicated 

 by the distribution of animals now obtain- 

 ing in Mexico. The high table-land of that 

 country contains an extension of the North 

 American fauna, while the tropical l(%v- 

 lands are South American. 



It is a truism to say that the Isthmus of 

 Panama is the strategic key to the zoolog- 

 ical relations of North and South America, 

 and yet it was not necessarily so, as other 

 lines of communication might conceivably 

 have been established. So far as our 

 knowledge extends, however, the geograph- 

 ical events in the history of the Isthmus 

 dominated the biological interrelations of 

 the continents which it now unites. When 

 the Isthmus was submerged. South Amer- 

 ica was in a state of nearly or quite com- 

 plete isolation and developed a highly pe- 

 culiar fauna, few elements of which were 

 shared with any other continent, and 

 which was as unique in its waj', though on 

 a higher plane, as is the Australian. It 

 was, so to speak, a highly interesting ex- 

 periment in evolution ; a great continent. 



