The philosophy behind Eugenics is with us still. Generally speaking, all of the original arguments in favor of Eugenics have become the same arguments in favor of birth control, abortion, euthanasia, and even cloning.
Chesterton understood this. But he understood it in 1910 (which is when he started writing this book, which was not published till 1922). As with so many other things, Chesterton saw exactly what we see. Only he saw it long before it happened.