134 MEANS OF DISPERSAL. [OH. Ill 



guide to their relationship. A species of bird or reptile 

 finding its way to an oceanic island might become darker 

 in colour. After an interval the parent stock might send 

 a colony to another oceanic island where the same modi- 

 fication might well occur, thus exactly reproducing the 

 first variety ; this view is supported by Dr Heilprin in his 

 text-book of geographical distribution, who controverts 

 Darwin's suggestion that the same variety cannot be 

 produced twice, owing to the fact that the parent form 

 which gave rise to the variety will be supplanted by its 

 improved offspring, by reminding the reader of the 

 tenacity of form possessed by certain animals, notably by 

 some of the Brachiopods, which have persisted unchanged 

 for many geological periods. Geology also seems to show 

 that forms do reappear after an interval of total absence ; 

 but this may be merely another instance of the lamentable 

 "imperfection of the geological record." Dr Heilprin 

 thinks that the tapir which occurs in the Oriental region 

 and again in the Neotropical may be really the offspring 

 of two distinct lines from separate tapiroid ancestors ; and 

 that therefore this discontinuity of range does not argue 

 an extinct parent form, which sent off offshoots from one 

 continent to the other. 



Other geological facts have to be considered which 

 will be deferred until the next chapter. 



Means of Dispersion of Animals. 



Animals can extend their range either by active or 

 passive migration. Both kinds of migration are hindered 

 by barriers of various kinds. 



