This sounds like it is going to be a very good joke. However, seriously, if you are the woman from Sealand who is desperate to gain Petra and access to her incredible talent in telepathy, the answer is that there is no difference. It is in Chapter Sixteen that she expounds her own philosophical approach to the existence of Waknuk and the particular way that they are trying to hold up the evolution of humanity with their strict notions of purity and their strictness in dealing with deviations. One of the messages of the novel is the way in which all life, including humans themselves, are evolving. Trying to stop that process from occurring is like trying to stop a rushing river with a tiny dam. Note how she describes Waknuk society:

"The Old People brought down Tribulation, and were broken into fragments by it. Your father and his kind are a part of those fragments. They have become history without being aware of it. They are determined still that there is a final form to defend: soon they will attain the stability they strive for, in the only form it is granted--a place among the fossils..."

Evolution is a fact of life, and in this novel Wyndham paints one picture of how humanity will evolve in the future following a terrible holocaust that has broken us into "fragments." Trying to resist such a natural and powerful process is futile, although the impact of this process has been massive for David and his group of telepaths.