DISTRIBUTION. 399 



the ways in which they do this the commonest is invasion 

 of territory. That tendency which we see in the human races, 

 to overrun and occupy one another's lands, as well as the 

 lands inhabited by inferior creatures, is a tendency exhibited 

 by all classes of organisms in various wa3^s. Among them, as 

 among mankind, there are permanent conquests, temporary 

 occupations, and occasional raids. Every spring an inroad is 

 made into the area which our own birds occupy, by birds from 

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

 to share the hips and haws of our hedges, and thus entail on 

 our native birds some mortality. Besides these regularly- 

 recurring incursions there are irregular ones; as of locusts 

 into countries not usually visited by them, or of certain 

 rodents which from time to time swarm into 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 rat, which must have taken to infesting ships 

 within these few centuries, furnishes a good illustration 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 Blatta 

 orientalis, shows us how these successful invasions last only 

 until there come more powerful invaders. Animals 



encroach on one another's spheres of existence in further 

 ways than by trespassing on one another's areas : they adopt 

 one another's modes of life. There are cases in which this 

 usurpation of habits is slight and temporary; and there are 

 cases where it is marked and permanent. Grey crows often 

 join gulls in picking up food between tide-marks; and gulls 

 may occasionally be seen many miles inland, feeding in 

 ploughed fields and on moors. Mr. Darwin has watched a 



