216 The American Naturalist. [March, 
sufficient cause for a similarity. While there is evidence that the 
basins of the great oceans have always been approximately the 
same as they are at present, the fringes of the continents, or the 
: platforms upon which continental islands rest, have, from time to 
time, suffered changes of elevation which, in many instances, have 
raised the intervening sea bottom above the surface of the water, 
and thus have brought about a means of communication for the 
life of continents and neighboring islands. In this case it is not 
necessary to call in the aid of various means of transportation 
across intervening bodies of water, straits and sounds, for the 
commingling of the two floras and faunas. Prehistoric -man in 
his early migrations, no doubt, was profoundly affected by a law 
of distribution similar to that which influenced animals and plants. 
Using islands as stepping stones he may thus, before he had the 
hardihood to attempt the ocean in navigation, have even passed 
from one continent to another at atime when portions of the 
shallower ridges of the ocean floor now submerged were elevated 
above the sea. In this way, for instance, he may have migrated 
on dry land from northern Europe to Greenland and then tothe 
North American continent, or, in a similar way, across what is 
now Behring Strait and the Aleutian Islands from Asia into 
Alaska. The possibilities of inter-communication between lands 
not separated by the abysses of the ocean for races of man, 
animals and plants are very great, and can only be read in the 
light of the great geological changes which have occurred on 
the margin of continents.in which elevation and depression have 
undoubtedly taken place. 
Islands which have become separated from the continents by 
submergence of the land, or by erosion and a cutting out of an 
intervening channel, preserve, in a measure, the fauna and flora 
of the adjacent continents, but they are independently affected in 
somewhat different ways by the struggle for existence of their 
inhabitants. New conditions may arise or old ones may persist 
which may or may not lead to the preservation of organic forms 
which have been exterminated on the adjoining continent. Inter- 
communication, however, between the two has always had an in- 
fluence in neutralizing the changes which might otherwise occur. 



