THE COMING OF THE BIRDS 83 



years? Probably none whatever. The bodies must 

 be deposited in certain rare conditions to be preserved 

 and "fossilized." They must be put, as a rule, in the 

 mud at the bottom of stagnant water. As the vast 

 majority die on land, they are not preserved at all. 

 Only a tiny fraction of the things that have lived on 

 the earth are thus preserved. 



So the story of evolution as it is written in the 

 rocks is very imperfect. Here and there it is almost 

 complete for a page or two. For instance, shell-fish 

 are easily preserved in lakes and there are cases 

 where we can trace gradual evolution for quite a long 

 distance. Sea-urchins are easily preserved in chalk- 

 ooze, and we can in many cases follow their gradual 

 evolution a long way in our chalk cliffs. But these 

 are exceptions. In other cases the bones are still 

 numerous enough to enable us to follow the gradual 

 evolution very fairly. We shall see this in the case 

 of the horse and the elephant. In most cases only 

 a specimen here and there in ages is preserved, by 

 some chance or other, and we are lucky to get these. 



That is the situation as regards the evolution of the 

 bird. Only one specimen (or two specimens of the 

 same bird) has been preserved in the rocks from 

 the hundreds of thousands of years during which 

 a branch of the reptiles was being transformed into 

 birds. It was found in certain rocks in Bavaria. 

 Millions of years ago the bird flew in the atmosphere 

 of Europe, which was then mostly under water, as 



