With the recent controversy over "intelligent design", I've been thinking about the theory of natural selection.
Adapt or die.
This is basically what agility is all about - figuring out what's working by doing a little bit, getting feedback, and adapting as necessary. Or to put it more simply:
Experiment. Inspect. Adapt. Repeat.
It's really that simple. Agile practitioners need to remember that the practices are helpful, but without inspecting what's working in a particular context, no practice should be followed blindly.
Two factors are going to force this issue in the near future. The acceptance of Scrum and XP by mainstream companies continues to rise, and there is an increasing trend towards globalization of the workforce. This will make distributed teams even more common, and will stretch the commonly held Agile "best practices" to their limits. Adaptation is the best strategy for keeping Agile methods relevant and effective for the needs of business.
Don't get me wrong, there are lots of very useful Agile practices that have been discovered, documented and validated over the last five to ten years and I follow many of them. But when the next technological ice age happens, I, for one, don't want to become extinct.