“Which do you think came first—the blood or the heart—and why? Did the heart in all these different species of fish, reptiles, birds, and mammals evolve before there were blood vessels throughout their bodies? When did the blood evolve? Was it before or after the vessels evolved? If it was before, what was it that carried blood to the heart, if there were no vessels? Did the heart beat before the blood evolved? Why was it beating if there was no blood to pump? If it wasn’t beating, why did it start when it had no awareness of blood? If the blood vessels evolved before there was blood, why did they evolve if there was no such thing as blood? And if the blood evolved before the heart evolved, what was it that caused it to circulate around the body? The marvelous human body (and the bodies of all the other creatures) consists of so many amazingly interdependent parts: a heart, lungs (to oxygenate the blood), kidneys (to filter wastes from the blood), blood vessels, arteries, blood, skin (to protect it all), etc. The intricate codependence of just the respiratory system and the circulatory system—not to mention all the other bodily systems—is difficult to explain.”

The Origin of Species: 150th Anniversary Edition (2009)