DISTRIBUTION. 317 



but certain tribes of them have taken to an aquatic existence 

 —swimming on the surface of the water and making continual 

 incursions beneath its surface ; and there are some genera 

 that have wholly lost the power of flight. Among mam- 

 mals, too, which have limbs and lungs implying an organiza- 

 tion for terrestrial life, may be named kinds that live 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 water, and somewhat more 

 fitted to it ; seals living almost exclusively in the sea, and 

 having the mammalian form greatly obscured ; whales 

 wholly confined to the sea, and having so little the aspect of 

 mammals as to be mistaken for fish. Conversely, sundry 

 inhabitants of the water make more or less prolonged ex- 

 cursions on the land. Eels migrate at night from one pool 

 to another. There are fish with specially-modified gills, and 

 fin-rays serving as stilts, which, when the rivers they in- 

 habit are partially dried -up, travel in search of better quarters 

 And while some kinds of crabs do not make land- excursions 

 beyond high-water mark, other kinds pursue lives almost 

 wholly terrestrial. 



Joining together these two classes of facts, we must regard 

 the bounds to each species' sphere of existence, as determined 

 by the balancing of two antagonist sets of forces. The tend- 

 ency which every species has to intrude on other areas, 

 other modes of life, and other media, is restrained by the 

 direct and indirect resistance of conditions, organic and inor- 

 ganic. And these expansive and repressive energies, vary- 

 ing continually in their respective intensities, rhythmically 

 equilibrate each other — maintain a limit that perpetually 

 oscillates from side to side of a certain mean. 



§ 106. As implied at the outset, the character of a region, 

 when unfavourable to any species, sufficiently accounts for the 

 absence of this species ; and thus its absence is not incon- 



