Untitled 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 superficial 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 everywhere like the overflowing waters of a great river in flood. § 3 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
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H. F. Osborn, R. S. Lull, Old land bridges, severe interglacial times