274 TROPICAL NATURE, AND OTHER ESSAYS. 



their presence will account for a certain proportion of 

 showy flowers being found there, such as the scarlet 

 Metrosideros, one of the few conspicuous flowers in 

 Tahiti. In the Sandwich Islands, too, there are forests 

 of Metrosideros ; and Mr. Charles Pickering writes me, 

 that they are visited by honey-sucking birds, one of 

 which is captured by sweetened bird-lime, against which 

 it thrusts its extensile tono-ue. I am also informed that 

 a considerable number of flowers are occasionally fertil- 

 ized by humming-birds in North America ; so that there 

 can, I thiDk, be little doubt that birds play a much 

 more important part in this respect than has hitherto 

 been imagined. It is not improbable that in Tropical 

 America, where the humming-bird family is so enor- 

 mously developed, many flowers will be found to be 

 expressly adapted to fertilization by them, just as so 

 many in our own country are specially adapted to the 

 visits of certain families or genera of insects.^ 



It must also be remembered, as Mr. Moseley has 

 suggested to me, that a flower which has acquired a 

 brilliant colour to attract insects might, on transference 

 to another country and becoming so modified as to be 

 capable of self-fertilization, retain the coloured petals for 



1 The probable influence of fertilization by birds on the flowers of the 

 Auckland Isles has been referred to at p. 238. Mr. Darwin, in his book on 

 Cross and Self-Fertilisation of Plants (p. 371), gives in a note numerous 

 cases in which birds are known to fertilise flowers, the most important being 

 that of several species of Ahutilon in South Brazil, which, according to Fritz 

 Miiller, are sterile unless fertilised by humming-birds. This proves, not 

 only that birds fertilise flowers in the same manner as insects, but that the 

 two classes of organisms have become so correlated as to be mutually neces- 

 sary to each other ; and it completely justifies us in imputing the fertilization 

 of flowers to flower-frequenting birds wherever these are present and suitable 

 insects are notoriously scarce, as is the case in so many of the islands here 

 referred to. 



