62 



ISLAND LIFE. 



[part I. 



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 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 ancient 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 owing to their nocturnal habits 

 and concealment in the densest forests. 



The Rise and Decay of Species and Genera. — The preceding 

 sketch of the mode in which species and genera have arisen, 

 have come to maturity, and then decay, leads us to some very 

 important conclusions as to the mode of distribution of animals. 

 When a species or a genus is increasing and spreading, it 

 necessarily occupies a continuous area which gets larger and 

 larger till it reaches a maximum ; and we accordingly find that 

 almost all extensive groups are thus continuous. When decay 

 commences, and the group, ceasing to be in harmony with its 

 environment, is encroached upon by other forms, the continuity 

 may frequently be broken. Sometimes the outlying species 

 may be the first to become extinct, and the group may simply 

 diminish in area while keeping a compact central mass; but 

 more often the process of extinction will be very irregular, and 

 may even divide the group into two or more disconnected 



