16. It can happen sometimes that even upon the imperfect planets through the effect of sunlight some more ancient species die out, whilst completely different ones come into being. But such cases on the planets are rare and the change and transition period requires a far lengthier period than upon the perfect sun-planet.
17. In this way upon your terrestrial body, several thousand tree, bush, plant and grass species have indeed perished, impressions of them having been found between layers of rock. Several primordial species of giant trees also have perished their wood now being found as hard coal. Likewise a large number of giant animals have completely vanished from existence as for instance the mammoth and a great many of yonder large winged amphibians still known as lizards.