Huxley had pointed out that "Archaeopteryx" was a mixture of reptile and bird features. Without the feathers and arms, its skeleton looked just like that of "Compsognathus". We now know its bone growth physiology was much slower than modern birds, and more like that of its dinosaur ancestors. This means it would take longer after hatching before it could fly. A modern precocial bird takes for 3–6 weeks from hatching to flying. In "Archaeopteryx" this milestone might have taken about 18 weeks. It might have taken two to three years to reach its final adult size. The evolution of the physiology of modern forms occurred later in the group's history. They have had over 140 million years to evolve after "Archaeopteryx". This is an example of mosaic evolution, where some characters in a transitional form are basal, while others are remarkably advanced.