Just wow. To look into his logic, there is are "divine laws" commonly believed but "mistaken", and "laws of nature" which he believes wholeheartedly. What makes the "laws of nature" any better than the "divine laws" apparently amount to convenience of the species (or himself?). It can be easily argued that there are no "laws of nature" since introducing a species into a new environment can wreck the ecosystem and cause the death of many species including the one introduced in the first place--not to mention the many mass-extinction events throughout natural history. If these things can be argued to have anything with respect to law it would be "lawlessness", and so arguing from the basis of "natural law", especially imputed to humanity which can possibly uniquely restrain itself for noble causes, is to argue not for better law, but for no law at all.
No doubt it is twisted stuff. But people believed it because it was backed by scientists. By the time the fourth edition of this book came out, more than 20,000 eugenic sterilizations were already performed in the US, and more than 50,000 in Germany. Sweden, I think, only stopped eugenic sterilizations far more recently. And, of course, many people still believe in the scientific validity of eugenics. Richard Dawkins, for example. And most of the Culture of Death crowd.