64 ISLAND LIJE i>Aiit t 



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 con- 

 tinuous area which gets larger and larger till it reaches a 

 maximum ; and we accordingly find that almost all exten- 

 sive groups are thus continuous. When decay commences, 

 and the group, ceasing to be in harmony with its environ- 

 ment, 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 portions. This is the more likely to 

 be the case because the most recently formed species, 

 probably adapted to local conditions and therefore most 

 removed from the general type of the group, will have the 

 best chance of surviving, and these may exist at several 

 isolated points of the area once occupied by the whole 

 group. We may thus understand how the phenomenon of 

 discontinuous areas has come about, and we may be sure 

 that when allied species or varieties of the same species 

 are found widely separated from each other, they were 

 once connected by intervening forms or by each extending 

 till it overlapped the other's area. 



Discontinuous Specific Areas, why Rare,- — But although 

 discontinuous generic areas, or the separation from each 

 other of species whose ancestors must once have occupied 

 conterminous or overlapping areas, is of frequent occur- 

 rence, yet undoubted cases of discontinuous specific areas 

 are very rare, except, as already stated, when one portion 

 of a species inhabits an island. A few examples among 

 mammalia have been referred to in our first chapter, but 

 it may be said that these are examples of the very com- 

 mon phenomenon of a species being only found in the 



