276 TEOPICAL NATURE, AND OTHER ESSAYS. 



fertilization in various degrees; so that we have no 

 difficulty in understanding how, as the insects dimin- 

 ished and finally disappeared, self-fertilization may have 

 become the rule, while the large and showy corollas 

 remain to tell us plainly of a once different state of things. 



Another interesting fact in connexion with this sub- 

 ject is the presence of arborescent forms of Compositse 

 in so many of the remotest oceanic islands. They occur 

 in the Galapagos, in Juan Fernandez, in St. Helena, in 

 the Sandwich Islands, and in New Zealand ; but they 

 are not directly related to each other ; representatives 

 of totally different tribes of this extensive order be- 

 coming arborescent in each group of islands. The 

 immense range and almost universal distribution of the 

 Compositse is due to the combination of a great facility 

 of distribution (by their seeds) with a great attractive- 

 ness to insects ; and to the capacity of being fertilized by 

 a variety of species of all orders, and especially by flies 

 and small beetles. Thus they would be among the 

 earliest of flowering plants to establish themselves on 

 oceanic islands ; but where insects of all kinds were 

 very scarce, it would be an advantage to gain increased 

 size and longevity, so that fertilization at an interval of 

 several years might suffice for the continuance of the 

 species. The arborescent form would combine with in- 

 creased longevity the advantage of increased size in 

 the struggle for existence with ferns and other early 

 colonists ; and these advantages have led to its being 

 independently produced in so many distant localities, 

 whose chief feature in common is their remoteness from 

 continents and the extreme poverty of their insect life. 



As the sweet odours of flowers are known to act in 



