Claim CB325:

A giraffe's heart must be quite large (it is over 24 lbs) to pump blood to the giraffe's head. A series of special one-way valves in the neck regulates blood flow, and there is a special net of elastic blood vessels at the base of the brain. Without these valves and elastic blood vessels, the blood pressure in the giraffe's head would be immense when it bends over, enough to cause brain damage. All of these features -- large heart, valves in the jugular vein, and wondernet of vessels -- must be in place simultaneously or the giraffe would die. They could not have evolved gradually.

Source:

Davis, Percival and Dean H. Kenyon, 1989. Of Pandas and People: The Central Question of Biological Origins (2nd ed.). Dallas, TX: Haughton, pp. 69-72.
Setterfield, Barry, 1998. Birds, beetles, and life. http://www.setterfield.org/essays/giraffe.html

Response:

  1. Darwin answered this claim in 1868 (206). The claim assumes that "gradually" must mean "one at a time." Not so. The different features could have (and almost certainly would have) evolved both simultaneously and gradually. Partial valves would have been useful for reducing blood pressure to a degree. An intermediate heart would have produced enough pressure for a shorter neck. A smaller net of blood vessels in the head could have handled the lesser pressure. As longer necks were selected for, all of the other components would have been modified bit by bit as well. In other words, for each inch that the neck grew, the giraffe's physiology would have evolved to support such growth before the next inch of neck growth.

References:

  1. Darwin, Charles, 1868. Variation of Animals and Plants Under Domestication, vol. 2, chpt. 20. London: John Murray. http://pages.britishlibrary.net/charles.darwin/texts/variation/variation20.html

Further Reading:

Gould, Stephen J., 1998. The tallest tale. In: Leonardo's Mountain of Clams and the Diet of Worms, New York: Three Rivers Press, 301-318.
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created 2003-5-18