In the following passage from his work Parts of Animals (circa 335 B.C.E.), Aristotle applies his classification of animals into blooded and bloodless, along with his notion of immanent teleology—the fact that every single thing in nature has an innate purpose—to explain why snakes have no feet:

The reason for the footlessness of snakes is that nature docs nothing in vain but in every case looks out for the best possible arrangement for each thing, saving its special entity and essence. Besides, as we have said before, no blooded animal can move at more than four points [that is, they can have no more than four feet]. Clearly, those blooded animals that are disproportionately long in relation to the rest of their body, as are snakes, cannot have feet. For they are not the...