486 THE JOURNAL OF GEOLOGY. 
It is now known that the Cretaceous of South America belongs 
to the tropical type, as does also that of Mexico. 
Still more recent observations have shown that the-Cretaceous 
of western North America belonged during the Neocomian or 
Knoxville to the Boreal or Russian type, but this resemblance 
soon died out, and that the Horsetown or Gault fauna was of 
central European type, like that of India. This kinship to the 
Indian faunas persisted until the end of the Cretaceous. Since 
Cretaceous time the marine provinces seem to coincide closely 
with the existing boundaries of the temperature and shore lines. 
MIGRATION. 
Causes of migration.—The lower forms of life produce many 
more young than can possibly come to maturity ; hence a lively 
competition ensues between those of like character as well as 
between those that are different. Under these conditions an 
equilibrium is attained, the field supports the maximum popula- 
tion, and all others migrate or perish. 
Thus migration would be a necessity even if the conditions 
remained always the same and the competition were always 
between members of the same fauna. But conditions never 
remain the same. The same causes drive individuals from one 
habitat and to another, and disturb the equilibrium there estab- 
lished. A disturbance taking place in an unimportant part of a 
province must therefore disturb the whole zodlogical region. 
And if we add to this changes in physical geography, we see 
that constant migration and intermigration must be a necessity. 
Any rising or sinking of shore lines would drive the inhab- 
itants from their dwelling places; each change in the outline of 
continents changes the direction of currents, and thus has a 
marked influence on migration. 
Extinction of faunas.—Today it is noticed that when the strug- 
ele for existence becomes too severe for a species it disappears. 
But the reason for this disappearance has always been uncertain. 
In geologic history a species has a certain length of life and dies 
out, never to reappear. The supposed contemporaneous extinc- 
