Page:The Outline of History Vol 1.djvu/70

 the lizards a few remain, the monitor of the Dutch East Indies is the largest; all the multitude and diversity of the dinosaurs have vanished. Only the crocodiles and the turtles and tortoises carry on in any quantity into Cainozoic times. The place of all these types in the picture that the Cainozoic fossils presently unfold to us is taken by other animals not closely related to the Mesozoic reptiles and certainly not descended from any of their ruling types. A new kind of life is in possession of the world.

This apparently abrupt ending up of the reptiles is, beyond all question, the most striking revolution in the whole history of the earth before the coming of mankind. It is probably connected with the close of a vast period of equable warm conditions and the onset of a new austerer age, in which the winters were bitterer and the summers brief but hot. The Mesozoic life, animal and vegetable alike, was adapted to warm conditions and capable of little resistance to cold. The new life, on the other hand, was before all things capable of resisting great changes of temperature.

Whatever it was that led to the extinction of the Mesozoic reptiles, it was probably some very far-reaching change indeed, for the life of the seas did at the same time undergo a similar catastrophic alteration. The crescendo and ending of the Reptiles on land was paralleled by the crescendo and ending of the Ammonites, a division of creatures like squids with coiled shells which swarmed in those ancient seas. All through the rocky record of this Mesozoic period there is a vast multitude and variety of these coiled shells; there are hundreds of species, and towards the end of the Mesozoic period they increased in diversity and produced exaggerated types. When the record resumes, these too have gone. So far as the reptiles are concerned, people may perhaps be inclined to argue that they were exterminated because the Mammals that replaced them competed with them, and were more fitted to survive; but nothing of the sort can be true of the Ammonites, because to this day their place has not been taken. Simply they are gone. Unknown conditions made it possible for them to live in the Mesozoic seas, and then some unknown change made life impossible for them. No genus of Ammonite survives to-day of all that vast variety,