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:
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.
