The Story of Evolution 103 



mammals, like duckmole and beaver, shore-frequenting seals and 

 manatees, and open-sea cetaceans, some of which dive far more 

 than full fathoms five. It is important to realise the perennial 

 tendency of animals to conquer 'every corner and to fill every 

 niche of opportunity, and to notice that this has been done by suc- 

 cessive sets of animals in succeeding ages. Most notably the 

 mammals repeat all the experiments of reptiles on a higher turn 

 of the spiral. Thus arises what is called convergence, the super- 

 ficial resemblance of unrelated types, like whales and fishes, the 

 resemblance being due to the fact that the different types are 

 similarly adapted to similar conditions of life. Professor H. F. 

 Osborn points out that mammals may seek any one of the twelve 

 different habitat-zones, and that in each of these there may be six 

 quite different kinds of food. Living creatures penetrate every- 

 where like the overflowing waters of a great river in flood. 



The Pliocene period was a more strenuous time, with less 

 genial climatic conditions, and with more intense competition. 

 Old land bridges were broken and new ones made, and the 

 geographical distribution underwent great changes. Professor 

 R. S. Lull describes the Pliocene as "a period of great unrest." 

 "Many migrations occurred the world over, new competitions 

 arose, and the weaker stocks began to show the effects of the 

 strenuous life. One momentous event seems to have occurred in 

 the Pliocene, and that was the transformation of the precursor 

 of humanity into man the culmination of the highest line of 

 evolution." 



The Pleistocene period was a time of sifting. There was a 

 continued elevation of the continental masses, and Ice Ages set 

 in, relieved by less severe interglacial times when the ice-sheets 

 retreated northwards for a time. Many types, like the mam- 

 moth, the woolly rhinoceros, the sabre-toothed tiger, the cave- 



