316 THE INDUCTIONS OF BIOLOGY. 



isms encroach on one another's spheres of existence, in fur- 

 ther 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 

 frequently join gulls and curlews in picking up food between 

 tide-marks ; and gulls and curlews may be occasionally seen 

 many miles inland, feeding in ploughed fields and on moors. 

 Mr Darwin has watched a fly-catcher catching fish. He 

 says that the greater titmouse sometimes adopts the practices 

 of the shrike, and sometimes of the nuthatch ; and that some 

 South American woodpeckers are frugivorous, while others 

 chase insects on the wing. Of habitual intrusions on the 

 occupations of other creatures, one case is furnished by the 

 sea-eagle ; which, besides hunting the surface of the land for 

 prey, like the rest of the hawk-tribe, often swoops down upon 

 fish. And Mr Darwin names a species of petrel that has 

 taken to diving, and has a considerable, modified organiza- 

 tion. These last cases introduce us to a still more 

 remarkable class of facts of kindred meaning. This intrusion 

 of organisms on one another's modes of life, goes to the ex- 

 tent of intruding on one another's media. The great mass 

 of flowering plants are terrestrial ; and are required to be so 

 by their process of fructification. But there are some which 

 live in the water, and protrude only their flowers above the 

 surface. Nay, there is a still more striking instance : on the 

 sea-shore may be found an alga a hundred yards inland, 

 and a pha3nogam rooted in salt-water. Among animals, 

 these interchanges of media are numerous. Nearly all 

 coleopterous insects are terrestrial ; but the water-beetle, 

 which like the rest of its order is an air-breather, has 

 aquatic habits. "Water appears to be an especially unfit 

 medium for a fly ; and yet Mr Lubbock has lately dis- 

 covered more than one species of fly living beneath the sur- 

 face of the water, and coming up only occasionally for air. 

 Birds, as a class, are especially fitted for an aerial existence ; 



