178 THE LESSON OF EVOLUTION 



only explanation appears to be that physical isolation 

 has preserved fortuitous variations which arose sub- 

 sequently to the isolation. 



Again, New Zealand and its neighbouring islands 

 possess more cormorants than any other part of the 

 world. The whole of America, north and south, has 

 twelve species ; there are seven in Asia ; six in 

 Africa ; five in Australia ; only three in Europe ; and 

 fifteen in New Zealand. Now the difference in the 

 external conditions cannot be so great in the New 

 Zealand seas as throughout America, from Canada to 

 Tierra del Fuego ; or as in Asia and Africa taken 

 together. Consequently we cannot suppose that the 

 specific characters have been induced by external 

 conditions. This is emphasised by the fact that one 

 species P. Carbo extends from Greenland through 

 Asia-minor and India to Australia and New Zealand. 

 The Australian birds of this species never have so 

 many white feathers on the head and neck as have 

 the birds in Europe, and they probably represent an 

 earlier form ; but no change has been made by the 

 external conditions. 



The real cause of the great number of species in 

 New Zealand and its islands is that these islands 

 lie at considerable distances from each other, and 

 were formed long ago , in the Pliocene period ; so 

 that they are older than most of the islands sur- 

 rounding the continents. Thus the New Zealand 

 cormorants have been isolated for a very long period 

 and have had time to vary. 



We can divide them into two groups ; one having 

 the legs and feet black, the other having them pale 



