Page:Popular Science Monthly Volume 10.djvu/231

Rh they go, you will observe are in favor of evolution to this extent, that they show that in former periods of the world's history creatures existed which overstepped the bounds of all existing classes and groups, and tended to fill up the intervals which at present exist between them. But we can go further than this. It is possible to fill up the interval between birds and reptiles in a much more striking manner. I do not think that this is to be done by looking upon what are called the Pterodactyls as the intermediate form between



birds and reptiles. Throughout the whole series of the mesozoic rocks we meet with some exceedingly remarkable flying creatures, some of which attain a great size, their wings having a span of eighteen or twenty feet or more, and these are known as Pterosauria, or Pterodactyls. We find these with a bird-like head and neck, with a vertebral column sometimes terminated by a short and sometimes by a long tail, and in which the bones of the skeleton present one of the peculiarities which are often considered to be most characteristic of birds—that of having pneumatic cavities, which make