248 WARFARE BETWEEN DIFFERENT SPECIES 



instinct responds freely to a wide range of stimuli 

 will be in a position to maintain a footing upon 

 the cliff. 



In trying to estimate the importance of the 

 hostility in its relation to the territory, we must 

 bear in mind that competition varies in different 

 seasons and in different localities. The surface 

 of the land is constantly undergoing modification, 

 partly owing to human and partly to physical 

 agency forests are cleared ; marshes are drained ; 

 the face of the sea-cliffs is altered by the erosion 

 of the waves ; here the coast may be locally 

 elevated, there locally depressed ; and so forth. 

 Many of these changes are slow and imper- 

 ceptible, many can be observed in our own 

 lifetime. The timber is felled and the under- 

 growth cleared in some wood, and the following 

 spring we notice a change in the character of 

 the bird population. Migrants which formerly 

 found in it no suitable accommodation now 

 begin to appear, and as the seasons pass by 

 and the undergrowth affords more and more 

 shelter for the nests and an increasing supply 

 of insect life, so their numbers increase until 

 the wood becomes an important breeding station, 

 resonant with the song of many individuals. 

 But slowly the growth increases ; the bushes 

 pass into saplings and the saplings into trees, 

 and the undergrowth then disappears just 

 as surely as do the migrants which can no 

 longer find there the conditions which they 

 require. 



Or, as an illustration of the effect produced 



