FAUNA AND FLORA OF CALIFORNIA. 369 



produced by a graceful waterfern, the Azolla Caroliniana, 

 that covered the water now monopolized by the luxuriant 

 but coarse weed — the Cotula. 



In both instances it was not so much direct interference 

 that changed the character of the vegetation, but a certain 

 inability of the native vegetation to conform to altered 

 conditions of things. 



There is a series of phenomena in the complex system 

 of changes that follows the occupation of new territories 

 which practically as well as theoretically is of great im- 

 portance. It is a series of changes that gradually establish 

 themselves in the relations between the vegetable kingdom 

 and the insect world. 



In new countries we find a certain harmony in these 

 relations. Undisturbed nature characterizes itself by a 

 greater variety of species than those exhibited in the 

 agricultural stage, when the battle for existence has begun 

 to thin out the original inhabitants of the soil. Another 

 peculiarity of this undisturbed state is a certain equilibrium 

 in the number of individuals of the different species, 

 amongst whom there is no preponderation of species, 

 otherwise than a very rare and even then a very transitory 

 one. 



The exclusive cultivation of agricultural plants and 

 domestic animals proves a first cause of long series of ef- 

 fects, of complications, modifying each other. For in- 

 stance, the planting of extensive orchards has favored an 

 increase of those insect species that live on different va- 

 rieties of fruit trees, and that formerly had a more or less 

 precarious existence on wild species of the same order, 

 mixed up with a forest vegetation of orders that do not 

 favor the multiplication of insect species, depending ex- 

 clusively on species related to certain fruit trees. 



This, of course, is changed after some time, when such 



2d Seb., Vol. V. ( 24 ] August 12, 1895. 



