114 THE WONDER OF LIFE 



(3) Of many of the smaller animals in a freshwater basin, 

 it is safe to say that they have been transported from some 

 similar haunt. The same or similar species occur in basins 

 separated by half the circumference of the globe. And 

 just as there are distinctive species of mammals and birds 

 in islands — e.g. the Orkney Vole and the St. Kilda Wren 

 — so there are distinctive species of crustaceans and fishes 

 in lakes, the explanation being in both cases the same, that 

 local variations have been helped by isolation to become 

 stable species. A very str ikin g instance may be found in 

 the large number of different species of char in British lochs. 



To explain the widespread faunistic uniformity in fresh- 

 water basins, Darwin referred to the agency of birds in 

 carrying organisms or germs of organisms from one fresh- 

 water basin to another, from one watershed to another; 

 to the wafting powers of the wind ; and to changes of land 

 level which may bring diHerent river beds into commimica- 

 tion. The capture of one river- valley by another running 

 in a different direction has often occurred, and may have 

 helped to distribute lacustrine types. It is probable, 

 however, that birds have been the chief agents in transport. 

 The startled duck that rises in a hurry from the water 

 often carries some entangled aquatic plants with it, and 

 animals on the plants. Thus another pond may be peopled. 

 In the clodlets of mud on the feet of birds many minute 

 animals have been found — Ostracods, Phyllopods and 

 Copepods (all sorts of ' water-fleas ' in short), Polyzoa and 

 Rotifers, and Nematode worms. No fewer than 537 

 plants were found represented in 6| ounces of mud, and 

 Darwin got eighty seeds to germinate from one clodlet 

 from one bird's foot. The role of birds as distributing 

 agents is well known to be very important for seeds, but 



