The Value of Paleontology. 7 
the Lower Eocenes of New Mexico and Wyoming and that of 
France are marked; similarity between the Pliocenes of the 
respective continents is evident. Descending in’ the scale, 
the parallels between the North American and New Zealand cre- 
taceous are very apparent, and the faune of the Carolinian and 
‘Wiirtemburgian Trias were the same. The great. interruptions in 
life marked by the appearance of great land areas near the close 
of the carboniferous and cretacecous periods are universally observed 
in the zodlogical areas of the Northern Hemisphere or Arctogza. 
The close of the cretaceous everywhere saw the end of Ammon- 
ites, Rudisites, and Sauropterygian and Dinosaurian reptiles, in 
spite, in North America at least, of physical continuity of deposits. 
Was this succession of interruptions of life universal over the 
globe, and do these trenchant lines justify the old assumption of 
repeated destructions and recreations of animal life? The for- 
mer question has already been answered in the negative by the 
explanation of the characters of the existing faunze of the southern 
hemisphere, where ancient types stili remain in considerable 
numbers. Moreover, some of the later periods of both North 
America and Europe are characterized by a large predominance of | 
forms of the corresponding southern continent. — It is indeed evi- 
dent that migration from the one continent to the other has taken 
place, and is amply sufficient to account for the abrupt changes in 
the life of each, without necessitating the intervention of creative 
acts. If glacial periods be dependent on cosmic movements, the 
obliquity of the earth’s axis to the sun would cause an alternation 
of cold periods in the opposite hemispheres. This is well known’ 
as a most potent cause of migration and extinction, and the 
known relations of the faunz would thus result from a greater or 
less alternate invasion of the one hemisphere by the life of the 
other. . 
But within the great time boundaries are distinct land faune, 
whose relation of distinction may not thus be accounted for. 
Thus the Miocene and Pliocene faunz of Western America are 
entirely distinct, but with corresponding members. The alter- 
. nate presence and absence of water areas adapted for the preser- 
vation of the remains of the animals will abundantly account for 
such minor interruptions. Such changing topography is well 
known as due to the slow vertical oscillations of the earth’s crust. 
