CLASSIFICATION AND DISTRIBUTION 383 



tory, prove of special advantage rather than of disadvantage, 

 and will be preserved, while the original type may be destroyed. 

 In the new locality, the species often assumes a form quite 

 unlike the original type, and becomes so differentiated that the 

 descendants can hardly be recognized as belonging to the 

 original species. This peculiar feature is especially noticeable 

 on some of the oceanic islands. Such islands may be hundreds 

 of miles from the mainland and only occasionally visited by 

 accidental stragglers; but they develop peculiar types of ani- 

 mals and plants distinctly their own, although originally com- 

 ing from the mainland. So different do they sometimes be- 

 come that they can hardly be recognized as close allies of the 

 mainland types. Although this change of type in new localities 

 is especially noticeable on oceanic islands, it undoubtedly 

 occurs on the continental areas as well. When a species 

 migrates into a new territory, and is placed under new condi- 

 tions of food and climate, and is in rivalry with new enemies, 

 modifications of the original type are sure to develop, and in 

 the end the form adopted is more or less different from that 

 of the original immigrant, which may be limited to its original 

 home. 



DISTRIBUTION OF ORGANISMS IN TIME: PALEONTOLOGY 



Geology discloses the fact that the earth's crust is made up 

 of a series of rocks which have been deposited during the long 

 ages of the past; and by the study of these successive layers 

 of rock we can learn various facts concerning the history of 

 the world during the time when the different strata were de- 

 posited. In many of these rocks we find remains of living or- 

 ganisms, called fossils, which comprised the life of the world 

 at various periods in its earlier history. The study of these 

 different fossil remains is known as paleontology (Gr. palaios 

 = ancient + on = being -f- logos = speech), and gives us an 

 outline history of organisms in the past. 



Paleontological history at best is very incomplete, since it 



