Prior to this work, one could consider all of Stapledon's prior works to be the same vision of the future. "Darkness and the Light" doesn't fit as easily with his other works, largely because Stapledon's predictions in "Last and First Men" were already far off the mark. However, predictions are not the main purpose of Stapledon's works, and if one sets aside that aspect, this could be viewed as a more detailed look at the First Men. That being said, the decision on whether or not to fit this book in with his previous works is one which is left to the reader, as this book can easily stand on its own as far as its content is concerned. Written in the darkest days of World War II, this book focuses on the concept of the Darkness and the Light, and a decision point in the future of which path the human race will take resulting from a Tibetan Renaissance which will take place. If it is successful, humanity moves towards the Light, but if it fails then humanity heads towards Darkness.
The book is divided into three sections. The first is titled "Crisis" and deals with the history (or future as it actually is) leading up to the Tibetan crossroads. This section covers the rise and fall of several world powers, culminating in one power led by Russia, and the other led by China. Between the two comes the rise of tiny Tibet with its Renaissance and its movement towards the Light, its presence causes a massive impact on both of the large empires which now must deal with this new threat to their corrupt way of life.
Section two is titled "Darkness" and it deals with the future history which occurs if the Tibet Renaissance fails. Stapledon details the creation of a world empire, followed by the total collapse of civilization through its various phases. He continues on to the end of Man which results from the world taking this particular path. This is a bleak future, one without hope and it results in the extinction of man and ultimately the destruction of Earth. This future is much shorter for Man than the future which results from the other path. There is a brief period in which a single world government is created, but it is only able to last a relatively short period before the collapse.
The third and final section is "The Light". In this section Stapledon details the future history if the Tibet Renaissance succeeds. Unlike the "Darkness" future which holds out little positive, the success of the Tibet Renaissance does not result in a future which is completely "Light". The survival of Tibet still leaves the two other empires in place, and there ensues a long struggle before the world unites, and even then there is a period where regional demands force difficult compromises upon the rest of the world. There are additional challenges even when the world appears to unite, as discoveries about the nature of the universe reveal.
There is no section to sum up these two futures; they are merely presented as two paths which humanity might take, and only humanity can decide which one it will take. Taken literally that decision is also a long way in the future, but one can and should think about this work and the period in which it was written. There are clear parallels to the situation faced by Stapledon's England at the time this was written. This aspect of the book is significant, and I think it is often lost on readers today. Even when factoring that aspect in, this book falls short of Stapledon's "Last Men in London" and "Star Maker" which have a similar style of narrative, but it does make the reading of this work more interesting and less like a reworking of the first part of "Last and First Men".