DERIVATION OF THE FLORA AND FAUNA 367 



represent true relics, descendants from the earliest settlers, whereas the other are 

 examples of progressive endemism? This is often said, but it is not, as far as I 

 can see, what Andrkws means. 



In the opinion of zoologists still infected with Lamarckian ideas environment 

 is the direct cause of new hereditary characters. Time after time we are told that 

 as soon as a "germ" happens to land on an island, it gives rise to something 

 new. However, this does not, Mavr says, imply that every little island is turned 

 into a centre of evolution (//p. 2 1 6). 



The small and usually rather isolated islands of Polynesia have not only not been 

 new centres of evolution, like the dalapagos or Hawaiian Islands, but, on the contrary, 

 there is good evidence that many of them are "traps". Species that reach these islands 

 are doomed to extinction. 



This is peculiar. There are many small islands stocked with both non-endemic 

 and endemic species, many of the latter stenotopic, it is true, but quite able to 

 hold their own as long the environment remains unchanged. It is a truism that, 

 if a newcomer lands on an island where, for climatic or other reasons it cannot 

 live, it is doomed to disappear pretty quickly, but there is no reason why, once 

 established and able to reproduce itself, it would become extinct as long as the 

 habitat does not undergo any change for the worse. It is generally known among 

 ornithologists, Mayr says, that island birds are very vulnerable. He continues: 



The recent considerations of Sewall Wright have given us a possible key to this curi- 

 ous phenomenon. Apparently in these isolated populations there is more gene loss than 

 gene mutation. The species are therefore adjusted to an exceedingly narrow limit ot 

 environmental conditions. They are unable to respond to any major change of conditions 

 and must die if such a change occurs, or are crowded out if competitors arrive. 



We cannot be sure that mutations, should they be induced, would do them 

 any good and we need not assume a "gene loss" to explain why organisms unable 

 to escape to a more favourable habitat are bound to become exterminated as a 

 result of "major changes of conditions". 



Zimmerman {2^^} is one of the prominent defenders of the idea that, in oceanic 

 islands, as exemplified by the Hawaiian chain, a small number of immigrants has 

 given rise to a comparatively rich fauna and flora; the proportion between genera 

 and species attains figures expected under continental conditions but certainly not 

 in oceanic islands. He admits that everything did not necessarily happen on the 

 present islands as we behold them, for differentiation may have begun on some 

 distant land and proceeded in the course of migration, with Hawaii as the terminus. 

 He picked out, as an example, a large curculionid genus ranging from Australia 

 to Micronesia and east to Marquesas; most islands or archipelagoes have their own 

 endemic species, which have developed on their respective islands. He thinks that, 

 if within a varying population, one pregnant female, not carrying the gene consti- 

 tution of the entire population, gets isolated on an oceanic island, she may stand 

 out as distinct from the average — and if this sequence of events "be accompanied 

 by conditions conducive to isolation and survival, rapid and diversified speciation 

 may follow" (p. 125). This process is repeated on island after island, and "the 



