What were we given Reason for?

In Kant's Groundwork of the Metaphysics of Morals, he presents an argument that we were given Reason for the cultivation of a good will (4:395-396). Kant had come to the conclusion that the only thing good in and itself is a good will, since all the other supposed virtues (like courage, moderation, wisdom, etc) can be used for unvirtuous ends. In order to get Reason a part in the construction of his moral philosophy he needs to prove that it is Reason that is there for the purpose of creating a good will. He starts by arguing that for any living thing, it is provide with no instrument (or tool or faculty) that is not best adapted to its end. In other words, if the end of my eyes is to see, they must be best constructed for the purpose of seeing. He then argues that our Reason, insofar at has a practical component, can only have been for the purpose of either preservation, welfare, happiness or the cultivation of a good will. Since the first three are best served by instinct, Reason therefore (by reductio ad absurdum) best serves the cultivation of a good will. Thus, it must have been given to us by our designer (God, presumably) to cultivate a good will. Thus, this is his plan for us, and we do best to abide by it, to develop this ethics here using Reason.

The first big assumption Kant makes is that all parts of us are best constructed for their ends. This was a frequent assumption at the time before Darwinism, but it is simply not well supported by empirical evidence at all. Animals aren't that well constructed, and humans certainly aren't that well constructed. We are amazingly complex and well-designed things, but to go out and argue that we are the best designed for our functions is obviously wrong. We've got imperfections and vestigial features and genetic disorders and so on. This is enough to dismantle Kant's argument, but the next argument is more interesting.

He next argues that instinct serves welfare, preservation and happiness better than Reason. That Reason is not the best faculty for ensuring preservation seems dubious. Instinct has that problem of being inflexible and not adapting well to unexpected circumstances. Reason is dependent on past experience, but it has the possibility of creative and surprising responses. Kant might respond, Reason requires a large brain, which is expensive in terms of time and resources, which might suggest it might not be best for survivability. We might respond, that the most Reason-heavy species, humans, is thriving through a wide range of environments on this planet, suggesting that the added expense may be more than compensated by the increased adaptability. And we could make similar argument for material welfare - Reason is better at maximizing material welfare for oneself and others.

But Kant also includes happiness as also better served by instinct than Reason, and he has some arguments. There is the argument about how thinking too much about how to attain happiness can get in the way of happiness. Or that looking too closely at one's happiness, can bring too much attention to the flaws. Or we might add to Kant's arguments that the ability to better consider alternatives increases the likelihood that you'll conceive of alternatives that are better than your current situation and be dissatisfied. In addition, Kant has to make the other side of the argument, that instinct would serve happiness better. If our instincts were specifically designed to only lead us toward things that would make us happy, then that certainly could be the case. It might not best serve material welfare or preservation, but perhaps instincts would lead us to greater happiness. Certainly, we don't seem like we are a species specifically designed for happiness, and that we could be better designed for that purpose if that was our end, and in such a case Reason would be superfluous.

Ultimately, the full argument falls flat, since living things are definitely not best designed for their ends, and Reason does seem to serve preservation and material welfare better than instinct. But Kant does raise an interesting possibility, that instinct and not Reason is the best road to happiness. Could Kant be in fact right?