i66 



THE NATURE BOOK 



There can be no doubt that the events 

 here shadowed have actually happened, 

 and the Ribble has taken advantage of its 

 superior powers to beggar its neighbours. 

 Examples of this kind of river piracy 

 might be quoted in great numbers from 



THE VICTORIA FALLS ON THE ZAMBESI. 



among our English rivers. One or two 

 must suffice. We have reason to believe 

 that at one time the Yorkshire rivers 

 having their origin in the Pennines — the 

 Swale, Ure, Nidd, Wharfe, Aire, Calder, 

 and Don — emptied themselves into the 

 North Sea by separate mouths. Along 

 the Vale of York, where the rocks are 

 softer than those of the Pennines or East 

 Yorkshire, the Don sent up a tributary 

 and successively captured the rivers to 

 form what is now the Ouse. Again, the 

 Thames once had its origin in the high 

 lands of Wales, and the Severn has 

 beheaded it. That is why the small 

 brooks which join to make the infant 



Thames flow through valleys with swings 

 altogether out of proportion to the 

 existing conditions. 



From this we see that the present courses 

 of our English rivers have not been deter- 

 mined by chance, but are the results 

 of the working of certain well- 

 known laws. They have been 

 evolved by processes of natural 

 selection. The best favoured 

 and strongest have survived, 

 while the weaker ones have 

 perished in the struggle for 

 existence.* It is evident that 

 as rivers are constantly bring- 

 ing material from the high 

 grounds to the sea, the mountains 

 are in process of being destroyed. 

 We can imagine that by long- 

 continued action they may be 

 levelled down to form a plain but 

 little raised above the level of 

 the sea. Over these plains 

 streams would flow in a feeble 

 condition, as though in the last 

 stages of decay. Now let us 

 suppose that the plain were 

 raised by earth movements to 

 form a plateau, or, what comes 

 to the same thing, so far as we 

 are concerned, that the sea lev^el 

 were to fall and leave the plain 

 elevated above the new sea level. 

 This would give the rivers a new- 

 lease of life. At first they would 

 tumble over the sea cliff as cas- 

 cades, and then, gradually work- 

 ing back through the })lateau, a 

 new period of levelling would 

 be commenced. 

 That is the story of the Zambesi and 

 the wonderful Falls discovered by Living- 

 stone in Central Africa. The Upper 

 Zambesi flows sluggishly through a great 

 plain only a little below the surrounding 

 country. Then it falls over a great 

 cascade 400 feet deep into a gorge which 

 continues for many miles towards the 

 coast. Similar features are to be seen 

 in other rivers in Africa, and even in 



* We must, of course, not overlook the fact that 

 the control of streams has been affected by other 

 geological considerations, more particuhirlv by events 

 which haj)pene(l during tlie Glacial pi'ri<id. At that 

 time a large portion of tlie Hritisli Isles was covered 

 with a mantle of ice. The normal drainage was 

 disturbed and in some cases pormaneutly changed. 



