Evolution

Life has a history. Every Species has changed over time, different species also share common ancestors. Species with common ancestors come from the same family tree or a phylogeny, this is where certain characteristics were passed on to certain descendants while other characteristics were passed to descendants on a different branch on the family tree. All phylogenetic trees are a hypothesis about the relationship among organisms. There are three domains these organisms fall under: Archaea, Bacteria, and Eukaryota. Homologies and analogies help to determine if animals have a common ancestor or not. For example, four limbs of tetrapods are a homologous characteristic Birds, mice, bats, and crocodiles all have four limbs, whereas sharks and other fish don't The ancestors of tetrapods evolved four limbs making this an inherited feature in their descendant. This presence means this group would be considered a homology.

http://evolution.berkeley.edu/evolibrary/article/evo_01
Biology Concepts and Connections - Pages 256 - 262 - Jane B. Reece; Jean L. Dickey; Martha R. Taylor; Kelly Hogan; Eric J. Simon; Neil A. Campbell.

http://uedata.berkeley.edu//media/2/51487_evo_resources_resource_image_252_original.gif

http://www.warrenhills.org/cms/lib/NJ01001092/Centricity/Domain/162/homologous%20tetrapod%20limbs2.jpg