Free On Monsters: An Unnatural History of Our Worst Fears by Stephen T. Asma

weirdness. Only your imagination need limit the permutations of viscera. While these beasts were being winnowed by the harsh environment, Empedocles imagines, humanoid creatures without sexual differentiation evolved out of mud. The final stage was the evolution of male and female genders.
    Aristotle did not like the idea that we were all monsters once and that “chance fitness” sorted us into our current zoological forms. But unliketoday’s creationists he did not object on religious grounds. Instead, he thought Empedocles was doing bad science. 19 According to Aristotle, Empedocles had the
causal
story exactly backward. “The process of development,” Aristotle explains, “is for the sake of the thing finally developed, and not this for the sake of the process. Empedocles, then, was in error when he said that many of the characters presented by animals were merely the results of incidental occurrences during their development.” 20 For Aristotle, the “essential form of a man” is what directs the developmental changes, bringing about a coherent biological process; the “essential form of an ox” similarly guides the process of oxen reproduction. Without the fixed essence, we would see reproductive chaos on a regular basis: monsters would be the norm rather than the exception. Since we don’t see that chaos, we can conclude that all development (embryological or evolutionary) must be working toward some fixed goal. If Aristotle had lived to see the birth of modern genetics, with its theory of stored hereditary information in the form of DNA, he would have celebrated it as a kind of formal recipe that guides growth and development. As it is, he had no such knowledge and made the best science he could with what he had.
    So we did not have a monstrous past, where parts of creatures accidentally clumped into organisms, because “parts” exist only as components of “wholes.” “That this is true,” Aristotle says, “is manifest by induction; for a house does not exist for the sake of bricks and stones, but these materials for the sake of the house; and the same is the case with the materials of other bodies.” 21 In contrast to this rather philosophical argument, Lucretius offers more mundane proof against monstrous ancestors: monsters would have needed the exact matching sexual organs in order to procreate, and that fit seems even harder to believe if chance is the only cause. 22 In the same passage, he offers an interesting argument for why centaurs could not exist. Lucretius points out that humans mature at a much slower rate than horses do; young foals are independent very quickly after birth, whereas humans are dependent on their mother for years. Consequently, the half-man half-horse would be ridiculous; when the back half of the monster was well grown, independent, and capable of running and jumping, the front half would not even be able to hold up its head. Lucretius uses this logic to eliminate the possibility of any hybrid monsters that are similarly discordant in terms of their developmental trajectories.
    Aristotle further reasoned that monsters are simply
mistakes
that occur when normal reproductive processes are interrupted or otherwise corrupted. Nature inadvertently creates monsters when the “essence” of the animal (its final or formal cause) is corrupted by wayward matter. In the same way that grammatical mistakes can creep into an author’s writings,Aristotle suggests, so, too, can biological mistakes creep into the purposeful direction of nature.
    Feminist historians have rightfully highlighted the gender implications of Aristotle’s biology. Aristotle argues that an essential form is uniform, the same for everyone in a given species. The form, being a kind of biological recipe, has a degree of specificity that differentiates one species of animal from another. Because
matter
is the same in every physical thing (earth, air, fire, and water, and their mixed “tissues”), it must