Circular Reasoning

"A few scientists (e.g., Williams 1970, 1973, Ball 1975, Ferguson 1976) have tried to show that evolutionary theory can predict. Their attempts, however, are concerned with the small changes of the special theory of evolution instead of the general one which is at issue and which is the main subject of the declaration published in the Humanist. These small changes do not prove large ones as Grene (1959) points out:

By what right are we to extrapolate the pattern by which colour or other such superficial characters are governed to the origin of species, let alone of classes, orders, phyla of living organisms?

The question of the testability of the general theory of evolution remains.
Basic textbooks of biology usually illustrate evolution using the concept of homologous structures. Here we have another example of circular reasoning that would not pass the prediction test for science. Homologous structures are defined as comparable parts of different life forms that have a common evolutionary origin. The forelimbs of a salamander, crocodile, bird, bat, whale, mole and man all have the same basic bone structure and are considered homologous. Similarity does not necessarily imply evolution. A student commenting to an evolutionary professor put it aptly: "They find a muscle in an animal and give it a name; in another animal they find a muscle in a similar position and give it the same name and then call it evolution." Darwin himself used the argument of similarity of structure to support evolution.

Lee (1969) points out that the argument is logically invalid:

He [Darwin] argued that morphological similarities were due to common descent and yet offered no further really acceptable evidence for common descent save morphological similarities. A circular piece of reasoning if there ever was one.

Hull (1967) makes the same complaint:

It is tautological to say that homologous resemblances are indicative of common line of descent, since by definition homologous resemblances are those resemblances due to common line of descent.

The same difficulty reappears when evolutionists attempt to classify living and fossil organisms so that their evolutionary relationships are revealed. One might select, for example, the group of invertebrates which most closely resembles the chordates and place the two groups near each other in a classification scheme. The classification is then often used as evidence for an evolutionary relationship."


(Ariel A. Roth, 'DOES EVOLUTION QUALIFY AS A SCIENTIFIC PRINCIPLE?'
from: http://www.grisda.org/origins/04004.htm )