Is the Rudin-Keisler order of ultrafilters linear?


Is it a well ordering?



Is the Rudin-Keisler order of ultrafilters linear?
This would be more suitable as a comment, but I do not have enough reputation points.
Among the first results that google spits out for Rudin Keisler is the paper 
Anatoly Gryzlov: On the Rudin-Keisler order on ultrafilters; <http://dx.doi.org/10.1016/S0166-8641(96)00109-5>
It claims that there are incomparable ultrafilters (with respect to R-K order) and provides several further references.


