THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 23 



currents and the free-swimming embryos of sedentary forms would be 

 carried in the same way. The relatively sudden appearance of a fauna 

 recognizable as originated in some distant region would at least lead to the 

 consideration of the possibility of ocean currents setting from the old to the 

 new locality and may betray the presence of a most important element in 

 the paleogeography of the time. 



Shore-lines extending parallel to lines of latitude would be more apt to 

 have similar conditions of temperature, unless currents changed the normal 

 conditions. The course of the Kuro Siwo brings warm water along the 

 south side of the Aleutian Islands and warm-water forms of the western side 

 of the Pacific are found far east. 



It is obvious that temperature is but one of the influences brought to 

 bear on a migrating fauna and great similarity of a migrant sedentary fauna 

 to its parent fauna must imply similarity in conditions other than tempera- 

 ture. The known physical conditions of one region may then with some 

 safety be applied to a second region in which the fauna is known but the 

 physical conditions are unknown. Ideal conditions for such similarity of 

 faunae would be found on an east-and-west coast, such as perhaps existed 

 on the southern shore of the North Atlantic continent. 



Terrestrial invertebrate fauna. — The wide distribution of midges, ephem- 

 erids, ants, etc., overtaken in their nuptial flights by violent winds is well 

 known. They may be blown far beyond their natural range, or caught in 

 bodies of water in enormous numbers. The accumulation of insects in the 

 water-laid ash-beds of Florissant, Colorado, is the record of such a catas- 

 trophe or series of catastrophes to insect life. It is very probable that 

 regions have been reached by insects, normally absent, through such forced 

 migrations, but it would be a faulty conclusion, if great numbers of fossils 

 were found suddenly introduced into a horizon from which they were 

 previously absent, that a great life migration had necessarily taken place; 

 the bodies may have reached these as the dead debris of some violent storm. 

 The movements of creeping land invertebrates would be far slower and less 

 liable to accidental acceleration; they would be far less liable to question 

 if used as evidence of land connections. 



Terrestrial vertebrate fatma.^The greater mobility of vertebrate animals 

 renders them far more fit to cope with minor changes in the environment and 

 makes them at once better and worse indices of surrounding conditions; 

 the highly developed power of locomotion and the ability to resist changes 

 of temperature introduces no small factor of uncertainty. Far more elusive 

 factors than the obvious ones so often cited above may determine the pres- 

 ence of such fossils far remote from their proper habitat. The migrations of 

 birds, for instance, are governed by what we call instinct, and a study of 

 the distribution of the remains of migratory birds would be a most formidable 

 problem to future paleontologists. Disregarding in large measure all bar- 



