CHAP. IV.] EVOLUTION THE KEY TO DISTEIBUTION. 



67 



groups of allied species distinguished from all other groups by 

 some well-marked structural characters, so families are groups 

 of allied genera distinguished by more marked and more im- 

 portant characters, which are generally accompanied by a pecu- 

 liar outward form and style of colouration, and by distinctive 

 habits and mode of life. As a genus is usually more ancient 

 than any of the species of which it is composed, because during 

 its growth and development the original rudimentary species 

 becomes supplanted 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 chaDges. 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 there- 

 fore be an indication of antiquity, and the more widely the 

 fragments are scattered the more ancient we may usually pre- 

 sume the parent group to be. A striking example is furnished 

 by the strange reptilian fishes forming 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 

 paradoxa is only known from the Amazon and some other South 

 American rivers. An allied species, Lepidosiren annectens, some- 

 times 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 Dipterus 

 evidently belong to the same group, while in North America 

 the Devonian rocks have yielded a gigantic allied form which 



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