Page 89 - The Evolution Impasse 2

Origin of the Avian Lung 87

This one-di rec tion al flow of air is In ad di tion, rep tiles have a di a -
main tained in breath ing in and breath - phrag mat ic re spir a to ry sys tem,
ing out by a com plex sys tem of in ter - while birds have no di a phragm. The
con nect ed air sacs in the bird's body, dif fer ing struc tures al so make ev o lu -
which ex pand and con tract in such a
tion be tween the two types im pos si -
way as to en sure a con tin u ous de liv ery
ble. John Ruben, an au thor i ty on re -
of air through the par a bron chi . . . The
spir a to ry phys i ol o gy, com ments:
struc ture of the lung in birds, and the
The ear li est sta ges in the der i va tion of
over all func tion ing of the re spir a to ry
the avi an ab dom i nal air sac sys tem
sys tem, are quite unique. No lung in
from a di a phragm-ven ti lat ing an ces tor
any oth er ver te brate spe cies in any
would have ne ces si tat ed se lec tion for a
way ap proach es the avi an sys tem.
di a phrag mat ic her nia in taxa tran si -
Moreover, in its es sen tial de tails, it is
tion al be tween ther o pods and birds.
iden ti cal in [all] birds. 84
Such a de bil i tat ing con di tion would
In short, a tran si tion from one
have im me di ate ly com pro mised the en -
type of lung to the oth er is not pos si -
tire pul mo nary ven ti la to ry ap pa ra tus
ble, be cause no "tran si tion al" lung and seems un like ly to have been of any
could func tion prop er ly. se lect ive ad van tage. 85