266 THE NATURALIST IN NICARAGUA. [Ch. XIV. 



not so easily leave. It is the answer to the question, 

 What became of the many peculiar tropical American 

 genera of animals and plants, when a great part of the 

 tropics was covered with ice, and the climate of the 

 lower lands must have been much colder than now. For 

 instance, the Heliconii and Morphos are a group of 

 butterflies peculiar to tropical America, containing many 

 distinct genera which, on any theory of descent from a 

 common progenitor, must have originated ages before the 

 glacial period. How is it that such peculiar tropical 

 groups were not exterminated by the cold of the glacial 

 period, or if able to stand the cold that they did not 

 spread into temperate regions on the retreat of the ice ? 

 I believe the answer is that there was much extermina- 

 tion during the glacial period, that many species and 

 some genera, as for instance, the American horse, did not 

 survive it, and that some of the great gaps that now 

 exist in natural history were then made, but that a 

 refuge was found for many species on lands now below 

 the ocean, that were uncovered by the lowering of the sea 

 caused by the immense quantity of water that was locked 

 up in frozen masses on the land. 



Mr. Alfred Tylor considers that the ice cap of the 

 glacial period was the cause of a great reduction of the 

 level of the sea amounting to at least 600 feet.* But if 

 we admit that the ice cap existed in both hemispheres at 

 the same time, and extended nearly to the equator, we 

 shall have to speculate on a lowering of the level of the 

 sea to at least 1000 feet. We have many facts tending to 

 prove that during the extreme extent of the glacial 



* " Geological Magazine," vol. ix. p. 392. 



