Thus we see that among birds and mammals limbs adapted for grasping,
flying, running, and swimming are all constructed upon the same basic
pattern. They share a fundamental similarity of structure which is
evidently entirely unconnected with the uses to which they are put. How can
we explain the origin of similarity of this kind, similarity which has no
relation to function--which, indeed, exists in many cases despite
dissimilar functions?
One way in which we might answer this question is that of the biologist who
first called attention to the fact that a basic pattern underlies all these
forelimbs. That was Cuvier, the eminent French comparative anatomist of the
past century. Cuvier was not convinced of the truth of evolution. He
believed that each species of animal had been created separately, the idea
usually referred to as the theory of special creation. But if species were
separately created how could similarities between them arise? Obviously
they would be similar if they were created to resemble one another. More
specifically, we might assume that in shaping forelimbs the Creator used a
certain pattern; when he created the hand of man he modified that pattern
in a certain way; when he created the wing of a bat he modified the pattern
in a different way; when he created legs adapted for rapid running he
modified the pattern in still a different way; and so on. According to this
theory there is no genetic relationship between man, bat, and horse; all
they have in common is that they were made by the same Creator. In much the
same way a dressmaker may use a pattern in constructing a dress of silk and
then, changing it somewhat if necessary, to construct a housedress of
cotton. The silk dress is not the "ancestor" of the cotton one, or even its
"sister" or "cousin" in any literal sense. Similarly, according to the
theory of special creation different species of animals are not genetically
related to one another even though they may exhibit similarites of structure.
Most modern biologists do not find this explanation satisfying. For one
thing, it is really not an explanation at all; it amounts to saying,
"Things are this way because they are this way." Furthermore, it removes
the subject from scientific inquiry. One can do no more than speculate as
to why the Creator chose to follow one pattern in creating diverse animals
rather than to use differing patterns. [note: this is no explanation at
all]
Hence most modern biologists explain the origin of similarities which have
no relation to similar functions in a different manner. *They are
convinced* that the similarity exists because the animals concerned
inherited the structure from an ancestor which they shared in common. We
have seen that the vertebrate forelimbs, for example, *appear to be*
modifications of a five-fingered (pentadactyl) limb having one upper-arm
bone (humerus), two lower-arm bones (radius and ulna), wristbones
(carpals), and metacarpals and phalanges arranged to form five fingers
(Fig. 3.2).
Why are such diverse limbs as those of man, bat, bird, whale, horse, and so
on, all modifications of this pattern? *The evolutionary explanation* is
that these animals all inherited the limb pattern from an ancestor which
had that pentadactyl limb in more or less typical form. When the
descendants of this ancestor took to life in the water, to locomotion
through the air, or to running over hard ground they made over what they
had in the way of limbs to serve the new functions. But *despite the
reconstruction necessary* the indelible traces of the inherited pattern
still remain. Thus, in contrast to the theory of special creation, the
theory of creation by evolution maintains that different animals are
related to each other in the sense of direct inheritance.
[note: this is an explanation]