GEOGRAPHICAL DISTRIBUTION 7 I 



know that the various classes, orders, families, etc., of animals 

 have appeared successively upon the stage. A group which 

 arose in the Coal Age followed lines of dispersal different from 

 one which was not evolved until Jurassic times, and post- 

 cretaceous creatures could not avail themselves of what assisted 

 their ancestors; and vice vcrsd. The Amphibia are bound abso- 

 lutely to the land and to fresh water; transportation across salt 

 water is not excluded, but must he accidental, and is not a case 

 of regular " spreading." Speaking generally, the older a group, 

 the more likely is it to be widely distributed. If it appears 

 sciittered, this may be due to extinction in intermediate countries 

 or to submergence of former land-connexions. 



There is great danger of arguing in a circle. It is one of the 

 most difficult tasks to decide in cases of great resemblance of 

 groups of animals between their being due to direct alhnity or 

 to heterogeneous convergence, or parallel development. It is tire 

 morphologist who is ultimately responsible for the establishment 

 of faunistic regions, not the systematist, least of all he who 

 accepts an elaborate classification, and then mechanically, mathe- 

 matically, by lists of genera and species, maps out the world. 

 Let us take an example. The Neotropical region and Mada- 

 gascar, but not Africa, are supposed to be faunistically related to 

 each other. In both namely occur Boa and Corallus amongst 

 snakes, Dendrobatinae amongst Eanidae, and of the Insectivora 

 Sdlcnodon in Cuba, Centetes in IMadagascar. More cases can 

 no doubt be found which would strengthen this resemblance, 

 perhaps in support of the startling view that Madagascar 

 and South America have received part of their fauna from the 

 famous Antarctica. But the value of the Insectivores has been 

 disposed of by their recognition as an extremely ancient group, 

 or as a ease of convergence, and the two genera are no longer put 

 into the same family as Centetidae. The Dendrobatinae {Man- 

 tella in Madagascar, the others in South America) are decidedly 

 not a natural group, but an instance of very recent convergence 

 (cf. p. 272). About the members of the ancient Boidae we do 

 not feel quite so sure. 



It. is therefore advisable to eliminate for zoogeographical 

 purposes groups about whiclr there can be any reasonable 

 doubt, otherwise we may argue that certain genera must con- 

 stitute a very old family, because they are now restricted to widely 



