DISTRIBUTION. 315 



with the influences, direct or more or less remote, of nearly- 

 all co- existing organisms. 



One general truth, indicated by sundry of the above illus- 

 trations, calls for special notice — the truth that organisms 

 are ever intruding on each other's spheres of existence. Of 

 the various modes in which this is shown, the commonest is 

 the invasion of territory. That tendency which we see in 

 the human races, to overrun and occupy each other's lands, 

 as well as the lands inhabited by inferior creatures, is a 

 tendency exhibited by all classes of organisms in all va- 

 rieties of ways. Among them, as among mankind, there are 

 permanent conquests, temporary occupations, and occasional 

 raids. Annual migrations are instances of this process in 

 its most familiar form. Every spring an inroad is made into 

 the area which our own fly- catchers occupy, by the swallows 

 of the South ; and every winter the fieldfares of the North, 

 come to share the hips and haws of our hedges with native 

 birds — a partial possession of their territory, which entails 

 on our native birds, some mortality. Besides these regularly- 

 recurring raids, there are irregular ones : as of locusts into 

 countries not usually visited by them ; or of strange birds 

 which in small flocks from time to time visit areas adjacent 

 to their own. Every now and then, an incursion ends in 

 permanent settlement — perhaps in conquest over indigenous 

 species. Within these few years, an American water- weed 

 has taken possession of our ponds and rivers, and to some 

 extent supplanted native water- weeds. Of animals, may be 

 named a small kind of red ant, having habits allied to those 

 of tropical ants, which has of late overrun many houses in 

 London. The case of the rat, which must have taken to 

 infesting ships within these few centuries, is a good illustra- 

 tion of the readiness of animals to occupy new places that 

 are available. And the way in which vessels visiting India, 

 are cleared of the European cockroach by the kindred BJatta 

 orientalis, shows us how these successfid invasions last only 

 until there come more powerful invaders. Organ 



