1.9 INVASION OF NEW ENVIRONMENTS n 



that the increasing complexity is related to the adoption of modes of 

 life continually more remote from the simple diffusion of substances 

 from the sea. Of course, even the earliest vertebrates had already 

 departed a long way from the first conditions of life and were quite 

 complex organisms. However, in the history of their life through 

 nearly 500 million years since the Ordovician period we can trace con- 

 siderable further changes in complexity. During this time vertebrate 

 life has left the sea to live in fresh water, on swampy land, and finally 

 on dry land and in the air. It has produced special types able to sup- 

 port life by such an astonishing variety of devices that we cannot 

 possibly specify them all. We shall only direct attention to a few, and 

 thus attempt to obtain an impression of the scheme of life of the vast 

 hordes of vertebrate animals, which, in one shape or another, have 

 swarmed and still swarm in the waters and over the earth. We shall 

 try to discern whether there is reason to suppose that all this variety 

 is related in some way to changes in the surrounding world and we 

 may therefore finish this introduction by a brief survey of the evi- 

 dences for climatic and geographical changes such as may have been 

 responsible for the changes in organic life. 



9. Changes of climate and geological periods 



9.1. Changes of level of the continents 



Changes of geography are mostly so slow that they cannot in them- 

 selves influence individual lives. On the other hand, nearly all living 

 things must be suited to daily and annual cyclical changes, unless they 

 live where no light enters. There is indirect evidence of further 

 changes in climate and geography, occurring with such long periods 

 that they are without appreciable effect on individual organisms, but 

 may greatly affect the history of the race. 



The idea of geographical change is made familiar by the fact that 

 coast-lines and river-courses have changed appreciably in historical 

 times. We are familiar with stories of destruction of some houses or 

 of a village by the sea, though it may come as a shock to learn that the 

 sea-level has changed so much that England and France were con- 

 nected by land 8,000 years ago, and that man-made instruments fished 

 up from the Dogger Bank show that it was an inhabited peat bog 

 6,000 years B.C. These changes in height of the land are signs of the 

 'diastrophic movements', which are major features of long-period 

 geological evolution. The earth forces that produce these movements 

 are still obscure but they lead to repeated elevation and sinking of the 

 land masses. The action of frost, wind, and rain continually breaks up 



