Human, Eagle and Bat

…….it is valuable to understand how this is the culmination of a process of comparative biology that starts with the most evident feature of organisms -- like the common bone structures in mammalian forelimbs that Darwin cited -- and proceeds to ever more minute details., the striking affinities of anatomy and physiology point clearly to relationships, but the exact character of the relationships, the determination of just which organisms are closer kin, depends on the history of modifications of the genetic material. ~ Page 53