PREFACE xi 
the application of natural selection to moral codes and social 
ideals. That moral code which works best, which fits the people 
who follow it to survive by making them strong and efficient is 
per se the best moral code. Whether we like it or not, such 
people will rule the earth and crowd out of existence other people 
who follow different codes which make for less efficiency. As the 
present writer has said elsewhere, one might as well say that he 
is unable to perceive a hygienic order of the universe merely 
because what he has believed to be hygienic practice does not 
secure him good health, or that he does not discover a harmony 
between the order of the universe and his supposedly hygienic 
practices. If morality is social hygiene, then we must apply the 
same test to our moral practices and beliefs that we are compelled 
to apply to our hygienic practices and beliefs. If our hygienic 
beliefs do not seem to work in matters of health, we will, if we 
are wise, change our beliefs, rather than try to change the uni- 
verse. Similarly, if our moral practices and beliefs do not seem 
to work, we must change our moral practices and beliefs rather 
than try to change the universe. 
It will require a much greater mental revolution to adjust our- 
selves to this new doctrine of social evolution than it ever took 
to adjust ourselves to a biological doctrine of evolution. The 
beliefs that were involved then were only traditional beliefs 
regarding the Creation. These beliefs were never very deep- 
seated, and a single generation was sufficient to bring about the 
discarding of the old and the adoption of the new; but our 
fundamental notions of right and wrong are very much older 
than the Biblical story of the Creation, and very much more 
deep-seated. To have to give up, for example, a cherished belief 
regarding democracy, or socialism, or individualism, or culture, 
or gentlemanly conduct, or as to what constitutes virtue, in order 
to square ourselves with the facts of the universe, will involve 
such a mental struggle that very few can be expected to come 
through it very successfully in any single generation. Neverthe- 
less, the process is going on. They will rule the world who are 
best fitted to rule the world by virtue of their strength and effi- 
ciency, not by virtue of the assumed beauty or persuasive power of 
