Very interesting question! I suppose the message of this frightening dystopian novel is that we must never become too arrogant or think that we are going to stay as we are. It is clear from the comments of the woman from Sealand that this has been the failing of the original human species, who are trying desperately to preserve society and humanity as they know it, and are implacably opposed to any form of development or evolution. Consider what she says:
Neither his kind, nor his kind of thinking will survive long. They are the crown of creation, they are ambition fulfilled - they have nowhere more to go. But life is change, that is how it differs from the rocks, change is its very nature. Who, then, were the recent lords of creation, that they should expect to remain unchanged?
Therefore, we must never become too arrogant as to think that we have reached the pinnacle of creation - for our form, and our world, can change drastically, showing us where we truly stand in the great scheme of things. Of course, Wyndham's novel seems just as relevant today with a very uncertain future for the world - either by nuclear war or environmental disaster. What would the fragments of humanity look like in the event of environmental oblivion? How would we react to no longer being the masters of the earth? It is these and such questions that this novel asks.