THE COMING OP THE BIRDS 87 



The mammals came for the same reason, at the same 

 time, from the same reptile world. But the Ice Age 

 melted away, as we saw, and the birds and mammals 

 had no advantage in their higher organization. Fur 

 coats, that you could not get rid of, were not a good 

 equipment in an age of warm perpetual summer. 

 Brooding over eggs was unnecessary, and unnecessary 

 labour is not encouraged in nature. So we are not 

 surprised that bird-remains are very rare. In rocks 

 which represent several million years of the Mesozoic 

 Era we have found only these two specimens of one 

 bird. 



Towards the end of the Age of Reptiles, in the 

 Chalk Period, we find a few others. They belong to 

 fully-formed birds of quite different types, and they 

 tell us that the bird world was now expanding. The 

 earth was cooling. Possibly the monstrous flying 

 reptiles, which would gulp down one of the early 

 birds like a dragon-fly, were being driven out of large 

 temperate regions, and making way for the birds. 

 As soon as they became lords of the air, they would, 

 like all new families, spread richly, and in different 

 directions. 



In one feature the birds of the Chalk Period still 

 bear the trace of their earthly ancestry. They have 

 teeth, either separate real teeth or toothed jaws. It 

 was a matter of evolution and progress for these teeth 

 to disappear. They meant weight at one end of the 

 flying machine; and in the course of development 



