266 Master Minds of Modern Science 



So came into being the reptiles which for ages formed the 

 only life on land, and these in turn evolved into the 

 mammals (warm-blooded animals) and the birds. 



The best and biggest change, says Sir Arthur, was that 

 which took life into the air, and he tells us that there were 

 no fewer than four different invasions of the air. First, 

 an invasion by insects, which have now become the most 

 plentiful of all living creatures ; secondly, an invasion by 

 flying reptiles, such as the pterodactyl (this was not a 

 successful invasion and lasted only for a time) ; third 

 came the bird invasion ; and lastly that of bats, warm- 

 blooded animals that took to flight. 



It is partly by observation of existing animals that we 

 are able to learn the long story of the evolution of species, 

 but our best books are the rocks and the fossils which 

 we find in them. We speak of the solid earth, yet 

 continents and mountain ranges are continually rising 

 and sinking. Rain and rivers are always carrying down 

 sand and gravel from the high grounds, and these, de- 

 posited elsewhere, harden into rocks. So the earth gets 

 skin after skin, and the rock record is like a library with 

 the oldest books on the lowest shelves. Some of the 

 shelves are broken, some volumes missing, yet practically 

 the whole story is there to read, and never a year passes 

 without fresh information coming to light. The story is 

 not finished, but still goes on, and if astronomers are right 

 it may continue yet for many millions of years. It is 

 only recently that scientists have proved their theory of 

 evolution — the slow, natural process of racial transforma- 

 tion — and the causes are still mysterious. 



" Life," as Sir Arthur writes, " continues to flow up 

 hill." 



There is not merely change, but constant improvement. 

 Nature is always making experiments. Some, like that 

 of the flying reptiles, fail and are abandoned. We will 

 conclude this short account of the origin of life with a 



