400 THE INDUCTIONS OF BIOLOGY. 



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 consider- 

 ably modified organization. The last cases introduce 

 a still more remarkable class of facts of kindred meaning. 

 This intrusion of organisms on one another's modes of life 

 goes to the extent of intruding on one another's media. The 

 great mass of flowering plants are terrestrial, and (aside from 

 other needs) are required to be so by their process of fructifi- 

 cation. But there are some which live in the water, and 

 protrude their flowers above the surface. ISTay, there is a still 

 more striking instance. At the sea-side may be found an alga 

 a hundred yards inland, and a phasnogam 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 extremely unfit 

 medium for a fly; and yet Mr. [now Sir John] Lubbock has 

 discovered more than one species of fly living beneath the 

 surface of the water and coming up occasionally for air. 

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

 but certain tribes of them have taken to an aquatic existence 

 — swimming on the surface of the water and making continual 

 incursions beneath it, and some kinds have wholly lost the 

 power of flight. Among mammals, too, which have limbs and 

 lungs implying an organization for terrestrial life, may be 

 named kinds living more or less in the water and are more 

 or less adapted to it. We have water-rats and otters which 

 unite the two kinds of life, and show but little modification; 

 hippopotami passing the greater part of their time in the 



