24 STRUGGLE AMONGST ANIMALS 



such as air and water, sunlight or shadow, house- 

 room and playground. According to Darwin, the 

 result of these circumstances is that on the average 

 the individuals better adapted to secure their neces- 

 sary share of these conditions of life survive longer, 

 and leave more progeny, than their less fortunate 

 relatives. 



No doubt the total effect of this internal struggle 

 amongst the individuals composing a species may be 

 to raise the general level of the species' capacity and 

 to permit one species to encroach on the ground of 

 another. It is astonishing, however, how little we 

 actually know of the influences that have been at 

 work when one species has replaced another or has 

 encroached on its territory. My friend and colleague, 

 Mr. R. I. Pocock, who has made a special study of 

 the wild dogs, jackals and foxes that occur in various 

 parts of the world, has reminded me of the case of 

 the Tasmanian wolf and the Australian wild dog or 

 dingo. The thylacine, or Tasmanian wolf, is the 

 fiercest of the marsupials or pouched mammals that 

 form the characteristic members of the fauna of 

 Australia. It is about the size of a large fox, brown 

 in colour with dark stripes, and is active and pre- 

 dacious. It is now limited to the island of Tasmania, 

 and indeed to the remote and mountainous parts of 

 that country, as its depredations on lambs and 

 poultry have made the settlers very unfriendly to 

 it. Formerly it was abundant on the mainland of 

 Australia, and teeth and skulls, belonging either to 

 the Tasmanian thylacine or to a closely allied species, 

 have been found in Queensland and New South 

 Wales, not only in the recent river gravels, cave 



