chap, iv.] EVOLUTION THE KEY TO DISTRIBUTION. 
f>7 
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 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 there- 
fore be an indication of antiquity, and the more widely the 
fragments are scattered the more ancient we may usu'ally 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 
