Adaptations to Environment 117 



wind-blown sand, and to slow changes of level, up and down. 

 The shore is a very crowded haunt, for it is comparatively narrow, 

 and every niche among the rocks may be precious. 



Keen Struggle for Existence 



It follows that the shore must be the scene of a keen struggle 

 for existence which includes all the answers-back that living 

 creatures make to environing difficulties and limitations. There 

 is struggle for food, accentuated by the fact that small items tend 

 to be swept away by the outgoing tide or to sink down the slope 

 to deep water. Apart from direct competition, e.g. between 

 hungry hermit-crabs, it often involves hard work to get a meal. 

 This is true even of apparently sluggish creatures. Thus the 

 Crumb-of -Bread Sponge, or any other seashore sponge, has to 

 lash large quantities of water through the intricate canal system 

 of its body before it can get a sufficient supply of the microscopic 

 organisms and organic particles on which it feeds. An index of 

 the intensity of the struggle for food is afforded by the nutritive 

 chains which bind animals together. The shore is almost noisy 

 with the conjugation of the verb to eat in its many tenses. One 

 pound of rock-cod requires for its formation ten pounds of 

 whelk; one pound of whelk requires ten pounds of sea- worms; 

 and one pound of worms requires ten pounds of sea-dust. Such 

 is the circulation of matter, ever passing from one embodiment or 

 incarnation to another. 



Besides struggle for food there is struggle for foothold and 

 for fresh air, struggle against the scouring tide and against the 

 pounding breakers. The risk of dislodgment is often great and 

 the fracture of limbs is a common accident. Of kinds of armour 

 the sea-urchin's hedgehog-like test, the crab's shard, the lim- 

 pet's shell there is great variety, surpassed only by that of 

 weapons the sea-anemone's stinging-cells, the sea-urchin's snap.- 

 ping-blades, the hermit-crab's forceps, the grappling tentacles 

 and parrot's-beak jaws of the octopus. 



