CHAP. IV EVOLUTION THE KEY TO DISTRIBUTION 69 



planted by more and more perfectly adapted forms, so a 

 family is usually older than its component genera, and 

 during the long period of its life-history may have survived 

 many and great terrestrial and organic changes. Many 

 families of the higher animals have now an almost world- 

 wide extension, or at least range over several continents ; 

 and it seems probable that all families which have survived 

 long enough to develop a considerable variety of generic 

 and specific forms have also at one time or other occupied 

 an extensive area. 



Discontinuity a Proof of Antiquity. — Discontinuity will 

 therefore be an indication of antiquity, and the more widely 

 the fragments are scattered the more ancient we may 

 usually presume the parent group to be. A striking- 

 example is furnished by the strange reptilian fishes form- 

 ing the order or sub- order Dipnoi, which includes the 

 Lepidosiren and its allies. Only three or four living- 

 species are known, and these inhabit tropical rivers situated 

 in the remotest continents. The Lepidosiren iKcracloxa is 

 only known from the Amazon and some other South 

 American rivers. An allied species, Zejnclosiren anneetens, 

 sometimes placed in a distinct genus, inhabits the Gambia 

 in West Africa, while the recent discovery in Eastern 

 Australia of the Ceratodus or mud-fish of Queensland, adds 

 another form to the same isolated group. Numerous 

 fossil teeth, long known from the Triassic beds of this 

 country, and also found in Germany and India in beds of 

 the same age, agree so closely with those of the living 

 Ceratodus that both are referred to the same genus. No 

 more recent traces of any such animal have been discovered, 

 but the Carboniferous Ctenodus and the Devonian Dip- 

 terus evidently belong to the same group, while in North 

 America the Devonian rocks have yielded a gigantic allied 

 form which has been named Heliodus by Professor Newberry. 

 Thus an enormous range in time is accompanied by a very 

 wide and scattered distribution of the existing species. 



Whenever, therefore, we find two or more living genera 

 belonging to the same family or order but not very closely 

 allied to each other, we may be sure that they are the 

 remnants of a once extensive group of genera ; and if we 



