Scientists refuse to study astrology, not because of prejudice or because there is a conspiracy afoot, but simply because there is not a shred of evi… - Mordechai Ben-Ari

Popper’s attempt to construct a methodology from falsification may have been flawed, but the concept is central in the demarcation of science from nonscience. No matter how strong one’s convictions, a true scientist will always allow for the possibility that her results may be falsified; if she denies this possibility or refuses to abandon or modify a theory in the face of repeated falsifications, you can be sure that you are dealing with pseudoscience, not science.

A scientific theory is a concise and coherent set of concepts, claims, and laws (frequently expressed mathematically) that can be used to precisely and accurately explain and predict natural phenomena.
A theory should include a mechanism that explains how its concepts, claims, and laws arise from lower-level theories.

Social Darwinism and eugenics are unscientific in two aspects. First, they commit the naturalistic fallacy, which is the assumption that what is, must be. The fact that the natural environment selects for reproductive advantage does not mean that we, as humans, should be forcibly selecting people according to some preconceived notions. The second nonscientific aspect of these movements was their narrow interpretation of the meaning of fitness. In evolution, this simply means fitness to survive and reproduce, not fitness according to some externally imposed criteria.
Darwin himself never engaged in these speculations, nor did he support these perversions of his theory.