摘要
There are about 9,000 species of birds living in the world. They have a more or less intimate association with human life. Birds are the only vertebrates that have dwelled throughout in the air, water and land. Unfortunately, little do we understand of their origins and evolutionary history. This is largely due to the poor preservation of the fossil ancestry. Like their modem descendants, the early ancestors had both an extensive range of life spans and very thin skeletons.
