Page 58 - The Origin of Birds and Flight

56 The Origin of Birds and Flight

often encounter in the media are mere fairy tales, as you shall soon see.
None of these provide the so-called missing link in the evolution of birds.
Gordon Rattray Taylor, himself an evolutionist, describes the theo-
ry’s inability to account for the birds’ origin:
. . . the number of modifications in reptilian structure which the birds
have managed to effect in order to adapt themselves for flight is so large
as to constitute a real problem and deserves our further attention. To
begin with, many modifications serve to reduce its weight. The bones
are hollow, the skull very thin. It has abandoned the heavy tooth-stud-
ded jaw for the light but rigid beak. The body is condensed into a com-
pact shape, the reptilian tail being abandoned, as also the reptilian
snout. The centre of gravity has been lowered by placing the chief mus-
cles beneath the main structure. Where organs are paired, like the kid-
ney, and the ovary, one has been sacrificed. The pelvis has been
strengthened to absorb (allow me the teleology) the shock of landing.
The legs and feet have been reduced to a minimum; the muscles operat-
ing them have vanished, to be replaced by muscles within the body. The
brain has been modified: a larger cerebellum to handle problems of bal-
ance and co-ordination, a larger visual cortex now that vision has
become more important than smell. Less obvious but even more
remarkable is the change in bodily metabolism.
To produce the energy for flight, the bird must consume a lot of fuel and
maintain a high [body] temperature. Not only do birds eat a lot, as any-
one who grows fruit or has seen the bullfinches systematically remove
every bud from a treasured shrub knows, but they have a crop in which
they can store fuel. So that it can handle more blood, the partitions in the
heart have been completed. The lungs too have not only been enlarged
but are supplemented by air-spaces within the body. In land creatures
like ourselves, much of the air in the lungs remains static; we exchange
only a very small proportion of it in a normal breath. The bird, by pass-
ing the inspired air right through the lung into the air-sacs, contrives to
exchange the lot with each breath. This system also serves to dissipate
the heat generated by the muscles during flight.