THE DISTRIBUTION OF ANIMALS AND PLANTS 407 



Thus we have at once in these continental regions a great and 

 suggestive example of the connection of geographical and geo- 

 logical distribution, the details of which are of the deepest in- 

 terest, and have not yet been fully worked out. One great 

 principle is, however, sufficiently established ; namely, that 

 the northern regions have been the birthplace of new forms of 

 land life, whence they have extended themselves to the south, 

 while the comparative isolation and equable climate of the 

 South American and Australian regions have enabled them to 

 shelter and retain the old moribund tribes. 



Those smaller portions of land separated from the con- 

 tinental masses, the islands properly so called, present, as 

 might be expected, many peculiar features. Wallace divides 

 them into two classes, though he admits that these pass into 

 each other. Continental islands are those in the vicinity of 

 continents. They consist of ancient as well as modern rock 

 formations, and contain animals .which indicate a former 

 continental connection. Some of these are separated from 

 the nearest mainland only by shallow seas or straits, and may 

 be assumed to have become islands only in recent geological 

 times. Others are divided from the nearest continent by very 

 deep water, so that they have probably been longer severed 

 from the mainland. These contain more peculiar assemblages 

 of animals and plants than the islands of the former class. 

 Oceanic islands are more remote from the continents. They 

 consist mostly of rocks belonging to the modern geological 

 periods, and contain no animals of those classes which can 

 migrate only by land. Such islands may be assumed never 

 to have been connected with any continent. The study of 

 the indigenous population of these various classes of islands 

 affords many curious and interesting results, which Wallace 

 has collected with vast industry and care, and which, on the 

 whole, he explains in a judicious manner and in accordance 

 with the facts of geology. When, however, he maintains that 



