CHAP. IV EVOLUTION THE KEY TO DISTRIBUTION 63 



epochs ; but the time will generally come when either 

 physical changes, or competing forms, or new enemies are 

 too much for it, and it begins to lose its supremacy. First 

 one then another of its component species will dwindle 

 away and become extinct, till at last only a few species 

 remain. Sometimes these soon follow the others and the 

 whole genus dies out, as thousands of genera have died out 

 during the long course of the earth's life-history ; but it 

 will also sometimes happen that a few species will con- 

 tinue to maintain themselves in areas where they are 

 removed from the influences that exterminated their 

 fellows. 



Cause of the Extinction of Species, — There is good reason 

 to believe that the most effective agent in the extinction 

 of species is the pressure of other species, whether as 

 enemies or merely as competitors. If therefore any portion 

 of the earth is cut off from the influx of new or more 

 highly organised animals, we may there expect to find the 

 remains of groups which have elsewhere become extinct. 

 In islands which have been long separated from their 

 parent continents these conditions are exactly fulfilled, and 

 it is in such places that we find the most striking 

 examples of the preservation of fragments of primeval 

 groups of animals, often widely separated from each other, 

 owing to their having been preserved at remote portions of 

 the area of the once widespread parental group. There 

 are many other ways in which portions of dying out groups 

 may be saved. Nocturnal or subterranean modes of life 

 may save a species from enemies or competitors, and many 

 of the ancient types still existing have such habits. The 

 dense gloom of equatorial forests also affords means of 

 concealment and protection, and we sometimes find in such 

 localities a few remnants of low types in the midst of a 

 general assemblage of higher forms. Some of the most 

 ancient types now living inhabit caves like the Proteus, or 

 bury themselves in mud like the Lepidosiren, or in sand 

 like the Amphioxus, the last being the most primitive of 

 all vertebrates ; while the Galeopithecus and Tarsius of the 

 Malay islands and the potto of West Africa, survive amid 

 the higher mammalia of the Asiatic and African continents 



