THE BRONTOSAUR AND ITS COUSINS 69 



severe chill of the face of the earth. There are 

 traces of glaciers here and there, but they do not 

 amount to very much, and we do not speak of an 

 Ice Age. But there is a real revolution in the life of 

 the planet. The Mesozoic Era ends with the time 

 when the great chalk-beds, composed of the " .shells " 

 of m3rriads of dead animalcules, are laid down on the 

 floor of the ocean. During this Cretaceous (Chalk) 

 Period there is a slow transformation of the world of 

 life. The great reptiles disappear; and most of the 

 smaller t3^pes, the survivors of that powerful dynasty, 

 retire into the tropics. This obviously means a chill. 

 The north is too cold for them, food is less abundant, 

 and leaving eggs to the care of nature is no longer 

 possible. Cold killed the reptilian monsters. 



At the same time, naturally, we find an expansion 

 of the birds and mammals. Not only are their fierce 

 enemies removed, but the conditions for which they 

 are particualrly fitted — cold climate and brisk move- 

 ment — return to the earth. It is a good example of 

 what we mean by "survival of the fittest." In the 

 Mesozoic Age the reptiles were fitter than the birds 

 and mammals. The latter were far superior in brain 

 and organization, yet they were the less fit for those 

 particular conditions. When the cold returned, the 

 conditions of the struggle were reversed. The huge, 

 heavily armed, and armoured reptiles were the 

 ** unfit." In spite of all their strength, they gave way 

 to tiny, rabbit-like creatures and birds. 



