INTRODUCING ANIMALS WITH BACKBONES
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A simple history of the development of the vertebrates begins with sea life and the first fish whose gristly cartilege replaced the backbone of today. Proceeding then to bony fish, walking fish, amphibians, the age of reptiles, birds and finally hairy land animals, the text traces development in this straight evolutionary line, giving examples of extant animals that are modern remnants of earlier species. If the order of progression seems arbitrary at times, it nevertheless serves to clarify the confusion and create a semblance of the evolutionary progress animals have shown.