What preserves the species.

This is a quote from Friedrich Nietzsches "The Gay Science" (Book 1:4) which I recently started to read:

The strongest and most evil spirits have so far done the most to advance humanity: again and again they relumed the passions that were going to sleep- all ordered society puts the passions to sleep- and they reawakened again and again the the sense of comparison, of contradiction, of the pleasure in what is new, daring, untried; they compelled men to pit opinion against opinion, model against model. Usually by force of arms, by toppling boundary markers, by violating pieties- but also by means of new religions and moralities. In every teacher and preacher of what is new we encounter the same "wickedness" that makes conquerors notorious, even if its expression is subtler and it does not immediately set the muscles in motion, and therefore also does not make one that notorious. What is new, however, is always evil, being that which wants to conquer and overthrow the old boundary markers and the old pieties; and only what is old is good. The good men are in all ages those who dig the old thoughts, digging deep and getting them to bear fruit- the farmers of the spirit. But eventually all land is exploited, and the ploughshare of evil must come again and again.