The second Greek philosopher to profoundly influence the thinking of the western world at the beginning of Darwin's century was Plato. Among other things, Plato devised the philosophy of Essentialism.
Plato, like Aristotle, believed that the universe was truly perfect. The problem with this was that imperfections in the world are all too obvious. So Plato decided that there were really two different realities to the universe. He called these the Essential universe and the Perceived universe.
In his view, the Essential universe was that perfect universe that he felt had to exist. The Perceived universe was a fašade, an overlay that we could see. All of the imperfections that are so evident to the eye would be a part of that Perceived reality; they wouldn't actually affect the perfect Essential reality beneath.
This notion carried over to all facets of the universe, including species of living things. By Plato's philosophy, every living creature also had two realities, the Essential and the Perceived. The Essential creature was perfect; the imperfections were all part of the Perceived (and somehow less real) creature. In the minds of the practitioners of the developing science that would become biology, this notion underlay assumptions about the nature of a species.
"Species" isn't an easy word to define. Today, most of our attempts at definition involve what happens when we try to mate two organisms, but none of those definitions is totally satisfactory. And yet, we have this feeling that species are real, concrete entities. To naturalists before the nineteenth century, the Essential creature was the species identity. The world might contain robins of all sizes, with all kinds of variation in exact coloration and proportion, but they were all robins. The implication of that statement is that, in their Essential realities, all of those robins are the same. All of the variation is part of the Perceived reality of the individual. Of course, when two robins mated, their offspring would inherit the exact same species identity (Essential reality) as the parents. Thus, it would seem literally impossible for a species to ever change all of the differences among members of the species are Perceived in nature, and thus not actually real.
When Darwin came up with his concept of Descent With Modification, he was specifically repudiating this notion about species. Darwin's view was that there were no "Essential" and "Perceived" aspects of creatures. The creature was as it appeared, with all of its quirks and deviations from the norm. Diversity was an intrinsic and absolutely real aspect of all populations, and not only did it affect the survival and reproductive prospects of the individuals in the population, but it could be passed on to their offspring. Thus, the population could change, and enough change would result in a new species.
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