4 io DISTRIBUTION IN SPACE AND TIME 



or an extensive area, or that it may be found in two or more 

 widely distant parts of the world, and be entirely absent from the 

 intervening regions. The last and perhaps the most interesting 

 case is technically described by speaking of a "discontinuous 

 area of distribution". Believing that existing species have been 

 evolved in course of time from other species, it is pretty obvious 

 that any sort of animal which occupies ^ a restricted area must 

 either have come into existence comparatively recently, or else 

 be an ancient form which has gradually lost ground and is pro- 

 gressing towards extinction. A good example of the latter state 

 of things is afforded by the Tuatara (Hatteria punctata), now 

 only to be found on some islets in the Bay of Plenty, off the 

 North Island of New Zealand, upon which larger land-mass we 

 know that it formerly existed. The evidence of geology also 

 proves that it is the last living representative of an order of 

 Reptiles (Rhyckocephala) which was once widely distributed and 

 dominant, being very likely the parent reptilian group from which 

 all the other orders took origin. 



In dealing with questions of distribution it is important to 

 remember that the outlines of land and sea have undergone many 

 changes in the course of the world's history. At various periods, 

 for example, the land-masses of the Old and New Worlds have 

 been connected together in the North, while Australia and the 

 East Indies are the surviving remnants of an extension of the 

 mainland of Asia. Comparatively recent union between land- 

 areas now distinct is often indicated by intervening shallow water, 

 more ancient union by deeper water. From this and other facts 

 we conclude that the British Isles were part of the continent of 

 Europe in comparatively recent times, while many ages have 

 elapsed since Madagascar was continuous with Africa, and the 

 connection of Australia with Asia was still more remote. On the 

 other hand, there are certain small islands isolated in mid-ocean, 

 such as St. Helena and Ascension, which probably never formed 

 part of any existing continent. Using this principle as a basis, 

 Wallace classifies islands as " continental ", e.g. the British Isles 

 and Madagascar, which once were united with adjacent main- 

 lands; and " oceanic", e.g. St. Helena, in which this has never 

 been the case. That such a view explains many of the features 

 of island faunas we shall presently see: the bearing of the former 

 existence of "land-bridges ", long since submerged, upon questions 



