With regard to natural law theorizing (what constitutes rational or natural behavior for an individual), contemporary Thomists are not, in my view, taking proper account of the fact that, in the higher species of animals, form does not drive the evolution of behavior, behavior drives the evolution of form.
Put another way, if a population of animals only took its cue to behavior from its existing form, its evolution would stall.
Whether it’s the flamingo’s “smile,” the panda’s thumb, or the bonobo female’s huge clitoris (which most characteristically gets rubbed on other females for pleasure and group bonding, countering male power in the species), behavioral variation–not playing to type or form–drives the evolution of form. Behavioral variation drives morphological change, not the other way around.
This is one of the cardinal rules of evolutionary biology. It’s been known for more than a century.
Put yet another way, if you behave differently from your given form, and that behavior proves beneficial to the species, it puts evolutionary pressure on the form to adapt to the new behavior (as with the bonobo’s ever enlarging clitoris).
Another example: before you’ll get shallow sea-dwelling creatures with their flat bellies oriented to the sand, you might first get fish swimming sideways. A disorder, you might say, but perhaps not from the vantage of evolution. In the right environment, it could prove to be an advantage that drives morphological reorganization.
Yet another example: before you’ll get a whale, you’ll first get a hairy land mammal oriented in an obsessive and uncharacteristic way (in relation to its form) to winning pleasure and food from the sea. The first step in the process might be little more than behaviorally dropping an aversion to water. The variations without the aversion might do better over time.
So when Thomas Aquinas proposed 700 years ago that the clues to one’s behavior should be read off of one’s forms–the penis is for reproduction only, etc.–he didn’t know Darwin. He didn’t know the role behavioral variation plays in driving the evolution of forms.
We now know that Aquinas had essence/accident turned exactly the wrong way around in relation to how a new species actually comes into existence. A lot of offspring have to play against type. There is no golden mean of form to conform to; there are only irreducible contingent variations in behavior along a continuum, many of them tugging at the most common usages of form in that species.
Nature doesn’t miss a bet. Behavioral variation is how nature keeps its bets open.
So when the natural law theorist says it’s irrational or unnatural to not play (or conform) to an average or characteristic type, he’s not taking proper account of how God, if God exists, plays against type–against form–to bring about new species.
Aquinas couldn’t have known this. Contemporary Thomists don’t have that excuse.
And this bears directly on irreducible sexual variation along a continuum. What’s rational and natural in sex cannot reasonably be said to be confined to a narrow and golden mean–the penis is for reproduction; the clitoris for stimulation only in the missionary position, etc. Evolution is more complicated than reading a narrow range of behaviors off of an attenuated and impoverished definition of form.
In architecture, form follows function. As a business or family’s needs change and behavior patterns change, rooms might be added to an existing building, and in a way that suits the surrounding environment.
In evolutionary biology, form follows behavior.
What I’m suggesting is that Thomistic essence/accident should be substituted with form follows behavior–and in humans, “form follows imagination.” No golden mean or average to conform to, but forms following contingent pursuits of imagination and passion.