326 DARWINISM AND THE PROBLEMS OF LIFE 



number of animals are separated spatially from the 

 great mass of their fellows, so that there can be no 

 further crossing. If these isolated animals come into 

 a different environment, selection will modify them in 

 a different direction from that taken by their distant 

 fellows. Remember the origin of land vertebrates. 

 As the fishes dispersed, passed up the rivers, and 

 reached their higher waters, some of them may have 

 swum into a piece of water that was not usually 

 connected with the rivers. When the water fell once 

 more, the wanderers were cut off from their species. 

 Selection had another effect in their new home. In 

 the hot season most of the water dried up, and this 

 was the occasion of the conversion of the swimming 

 bladder into lungs. 



It must have happened very frequently during the 

 millions of years of the earth's history that wandering 

 animals were cut off from their fellows. In this way 

 floods and evaporation must have given occasion over 

 and over again to the formation of new species of 

 aquatic animals. Land-animals also must have been 

 separated by inundations. Suppose, for instance, that 

 the overflow of the Rhine into a certain valley killed 

 all the deer in it, and only left survivors on the 

 mountains at each side. When the water subsides 

 again, the deer will remain in their separate localities, 

 because we know that they keep always to a rather 

 restricted area, and do not willingly leave it. Gradu- 

 ally, the increase of the animals on the mountains will 

 bn-ing some of them down into the valley once more. 

 But if they had a different environment on one range 



