What I wrote:
According to evolutionary theory, the homologous structures depicted above are evidence of common descent, and are accounted for by it. All forelimbs are derived from a common ancestor whose forelimbs showed the basic structure in common between the four examples above, and the differences between the examples are accounted for by subsequent alterations, thru mutations, in the specific forms and proportions of the components of the limbs. That is to say the reason these forelimbs all show the same basic pentadactyl structure is that they were all descended from a pentadactyl ancestor, and developmental constraints (which themselves are merely historically contingent and not functional in origin) explain why the pentadactyl trait has been retained in multiple lineages over time.
It is important to note that, according to the above account, there is no functional explanation for the observed homology. Rather, this is a result of contingent factors that occurred in the history of the evolution of life on earth. If the “tape” of life were rewound to sometime before the existence of vertebrate organism, there is no reason to believe that anything resembling vertebrates with pentadactyl forelimbs would again arise.
Denton’s account starts from the (correct) premise that this homology has no functional explanation. He proposes a theoretical frameworks that he calls “structuralism”. According to this idea, the “homology” between those four limbs is an expression of deep, fundamental laws of physics and chemistry. His position is that, if life had arisen on other planets, we would still find vertebrates with pentadactyl limbs, for the much the same reasons we would expect to find quartz crystals under the geological conditions that produce them here on earth.
What do you think I got wrong?