Dogs Have Arms and Legs

In The Origin of Species, Charles Darwin has a section called "Morphology” in which he discusses homology — the fact that basic body structures seem to be of the same type across very different groups of animals, even though they may differ in form or function. 

Darwin notes the similarities in bone structures between the limbs of humans, wings of birds, and the paddles of porpoises used for swimming. Similar bones were placed in a similar order, and seemingly started from a similar pattern as evidenced in embryology.

Darwin suggests this phenomenon indicates a shared ancestor whose original body-plan has been modified over time. This observation supported the claim that each species had not been uniquely and independently created.

Dogs are no exception to Darwin’s observation.  

Though we talk of dogs having front and back legs, dogs are built like us if we walked on all fours, with their legs in the back and their “hands and arms” in the front.

What is true of dogs is also true of such diverse animals as dinosaurs, horses, emus, bats, fish, lions, seals, and giraffes.

The back “legs” are legs, but the front “legs” (or wings, flippers, or fins) are analogous — and morphologically similar — to our arms and hands.