MARINE AND TERRESTRIAL. 233 



and ill-developed ; one species after another ceases to ar>. 

 pear, as the habitat becomes wholly unfavourable to it ; until 

 at length we find, instead of the brown, rootless, flowerless 

 fucoids and confervas of the ocean, the green, rooted, flower- 

 bearing flags, rushes, and aquatic grasses of the fresh water. 

 Many thousands of years have failed to originate a single in- 

 termediate plant. And such, tested by a singularly exten- 

 sive experience, is the general evidence. 



There is scarce a chain-length of the shores of Britain and 

 Ireland that has not been a hundred and a hundred times ex- 

 plored by the botanist, — keen to collect and prompt to re- 

 gister every rarity of the vegetable kingdom ; but has he 

 ever yet succeeded in transferring to his herbarium a single 

 plant caught in the transition state 1 Nay, are there any of 

 the laws under which the vegetable kingdom exists better 

 known than those laws which fix certain species of the algae 

 to certain zones of coast, in which each, according to the 

 overlying depth of water and the nature of the bottom, finds 

 the only habitat in which it can exist 1 The rough-stemmed 

 tangle (Laminaria digitata) can exist no higher on the shore 

 than the low line of ebb during stream-tides ; the smooth- 

 stemmed tangle (Laminaria saccharina) flourishes along an 

 inner belt, partially uncovered during the ebbs of the larger 

 neaps ; the forked and cracker kelp-weeds (Fucus serratus 

 and Fucus nodosus) thrive in a zone still less deeply covered 

 by water, and which even the lower neaps expose. And at 

 least one other species of kelp-weed, the Fucus vesiculosus, 

 occurs in a zone higher still, though, as it creeps upwards on 

 the rocky beach, it loses its characteristic bladders, and be- 

 comes short and narrow of frond. The thick brown tufts of 

 Fucus canaliculatus, which in the lower and middle reaches 

 of the Lake of Stennis I found heaped up in great abundance 

 along the shores, also rises high on rocky beaches, — so high 

 m some instances that during neap-tides it remains uncovep- 



