Is the survival of the species an irrational purpose? Yes, but nevertheless a useful one. If the species survives, wisdom may survive with it.
Is it then rational to desire the survival of wisdom? Well, this purpose most resembles reason, and is therefore the natural purpose for a mind that values reason. It is a purpose that remains after thinking has run its full course and exhausted itself. Such a purpose is neither rational nor irrational, and is not prone to common analysis.
Upon reaching the end of reason, reason itself is abandoned, just as one would abandon a boat when one has reached the farther shore. Purpose then arises spontaneously. Yet this is not to say it arises without thinking, without any effort or desire.