"We compared the morphological distance between ... two birds with that between a seal and a cat ... because of the following statement made by an eminent comparative zoologist: '...birds of today despite their varied plumage, songs and habits, are very similar to one another in their structure. They are divided into many orders; but the differences, for example, between a hummingbird and an albatross are much less than those between a seal and a cat.... The different bird orders have, in general, no more differences between them than exist between families in other classes of vertebrates, and anatomical differences between bird genera are often so slight that fossils are hard to place.'" [Alfred Romer, 1966. "Vertebrate Paleontology," 3rd ed., p. 468].