CLIMATIC CHANGES 353 



tell us nothing of the land areas which have disappeared beneath 

 the sea. In this connection fossils are of great assistance, for, in 

 certain instances, the distribution of marine fossils points to the 

 presence of land barriers to migration which no longer exist, while 

 the fossils of land animals may demonstrate the former existence 

 of land bridges between regions which have long been separated 

 by water. Thus it may be shown that North America was fre- 

 quently and for long periods of time connected with Asia across 

 Bering's Sea, and that its union with South America is of geologi- 

 cally late date. 



(3) As Evidence of Climatic Changes. — The remarkable cli- 

 matic changes through which various parts of the earth have 

 passed are indicated by fossils. Indeed, with the exception of 

 glacial marks, and ice-formed deposits, fossils offer almost the 

 only trustworthy evidence available as to these changes of climate. 

 Thus, when we find in the rocks of Greenland the remains of ex- 

 tensive forests of such trees as now grow in temperate latitudes, 

 the only possible inference is that Greenland now has a far colder 

 climate than when those forests existed. The same conclusion 

 follows from the presence in the rocks of Wyoming and Idaho of 

 great palm leaves and other subtropical plants associated with the 

 bones of crocodiles and other reptiles, such as live only in warm 

 regions. In deposits of a far later date occur bones of the rein- 

 deer in southern New England and in the south of France, walrus 

 bones in the sands of New Jersey, and those of the musk-ox in 

 Arkansas ; all of which shows that at one time these regions had a 

 far colder climate than at present. 



The evidence as to climatic changes which is presented by fos- 

 sils must, however, be treated with great caution, because even 

 nearly allied species often have entirely different habits and 

 flourish in quite different climates. Most fossils belong to extinct 

 species, as to whose climatic relations we have no knowledge. 

 Before any conclusion concerning changes of climate can be re- 

 garded as established, we should have the testimony of species 

 still living, or, if that is not possible, the evidence must be drawn 

 from large assemblages of different kinds of animals and plants. 

 2 A 



