DISPERSAL 151 



forming any particular habitat hardly ever remains for a long 

 time in the same spot, owing to the process of ecological succes- 

 sion which is almost everywhere at work ; and since succession 

 takes place fairly slowly, any habitat carries along with it a 

 complete set of animals, which thus gradually extend or change 

 their range. This extension of range takes place slowly and 

 quietly, and the species may end up by living in the same 

 habitat but in an entirely different locality, as it were, " without 

 knowing it." A simple example of this sort of thing is the 

 southward extension of arctic species of animals during an ice- 

 age in Europe. The animals must in many cases have changed 

 their range with almost imperceptible slowness, as the physical 

 conditions or vegetation gradually shifted southwards ; often 

 the process would be irregular, sometimes backwards and some- 

 times forwards, and over and over again patches of vegetation 

 would be completely destroyed and wiped out. But on the 

 whole the tundra zone, and the forest belt, would move south, 

 and with it the fauna, until the latter had penetrated hundreds 

 of miles south of its original home. The point is that such 

 spreading would not necessarily be accompanied by any 

 violent special migrations, except in so far as the latter were 

 normally found for various other reasons. 



Another example which illustrates the same thing on a 

 smaller scale is the formation of '' ox-bow " ponds in river 

 valleys. On the flood plain of a place like the Thames Valley 

 there are a number of ponds which owe their origin to the un- 

 easy movements of the river in its bed. Arms, loops, or back- 

 waters of the river become cut off as the latter shifts its course, 

 and in the ponds so formed there are usually a number of 

 molluscs or fish which would not have been able to reach the 

 pond if the whole environment had not shifted. The examples of 

 ice-ages and ox-bow ponds are only special cases of a universal 

 and very important way in which species spread over the 

 surface of the earth. The process consists of a combination 

 of the migration of the environment and of the local move- 

 ments of the animals in pursuit of their normal activities. 

 Any new extension of a habitat, such as the edge of a pine wood, 

 is immediately filled up in an almost imperceptible way. 



