The two aspects that difference and change resolve themselves into when expressed in an evolutionary formula are, in the inorganic world, equilibrium, and, in the organic world, adaptation. Of course, equilibrium also applies to the organic world, I merely put it this way for the purpose of clarity. Now, if we confine our attention to the world of animal forms, what we have expressed, primarily, is the fact of adaptation. If an animal is to live it must be adapted to its surroundings to at least the extent of being able to overcome or to neutralize the forces that threaten its existence. That is quite a common-place, since all it says is that to live an animal must be fit to live, but all great truths are common-places--when one sees them. Still, if there were only adaptations to consider, and if the environment to which adaptation is to be secured, remained constant, all we should have would be the deaths of those not able to live, with the survival of those more fortunately endowed. There would be nothing that we could call, even to please ourselves, either progress or its reverse. Movement up or down (both human landmarks) occurs because the environment itself undergoes a change. Either the material conditions change, or the pressure of numbers initiates a contest for survival, although more commonly one may imagine both causes in operation at the same time. But the consequence is the introduction of a new quality into the struggle for existence. It becomes a question of a greater endowment of the qualities that spell survival. And that paves the way to progress--or the reverse. But one must bear in mind that, whether the movement be in one direction or the other, it is still the same process that is at work. Evolution levels neither "up" nor "down." Up and down is as relative in biology as it is in astronomy. In nature there is neither better nor worse, neither high nor low, neither good nor bad, there are only differences, and if that had been properly appreciated by all, very few of the apologies for Theism would ever have seen the light.