In this essay we introduce an interesting debate that arose in the nineteenth century, with the participation of outstanding biologists. This debate started by the observation of the very specific present in the animal kingdom, some looked at the feet of many animals, the wings of many birds, each bone, each structure and hence there were two explanations for their origin:
- Existence Conditions: This position, defended mainly by Georges Cuvier, a great paleontologist but also a staunch creationist (position which holds that species have been independently created and not evolved from a common ancestor), said that different species have been created for various "conditions", that is why the wing of a penguin is used mostly for swimming, the wing of a hawk used to efficient flight, the leg of a horse used to run long distances, etc. In short, Cuvier was based mostly on differences structures between animals with a teleological view (based on function).
- Unit Type: This position was defended mainly by Étienne Geoffroy, which was supported by many evolutionists of the time (including Charles Darwin). They focused more on the similarities than differences. They noted, for example, that all vertebrate forelimbs have the same bone structure as follows:
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| A typical forelimb with estilópodo, and self-power zeugópodo | | | |
therefore concluded that there were no real conditions of existence, what existed was a "Unit Type", a common pattern which is built on different varieties depending on the ecological context of the animal. In addition, these varieties can be explained by natural selection, without having to resort to a supernatural agent to generate them.
Over time, the position lost Cuvier credibility and position and Darwin Geoffroy been gaining more weight, and that what they observed, after all, was how evolution generates specificity using always the same elements (which, as we have seen, occurs at the molecular level).
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