It seems, then, according to the optimistic view set forth in the previous chapter, that Evolution is necessarily Progress, and progress is movement in the line of our moral aspirations produced _ad infinitum_. The changes that are and always have been taking place are and always have been changes for the better; the forms of existence which incessantly succeed one another necessarily develop from lower to higher, from good to better. And this conclusion is not a matter of religious faith, but of scientific necessity. The only forces and causes that it presupposes are those which we see and feel at work every day around us. For the reconstruction of the past history of the earth's surface, geology only requires to assume the operation during infinite past time of those agencies which at this moment may be seen to be slowly changing the face of the earth. The cooling of the earth's surface follows the same laws, and can be calculated with the same certainty as the cooling of a red-hot poker. The law of gravitation, which determines the movements of the heavenly bodies, is equally exemplified in the fall of an apple to the ground. In fine, the universe consists of bodies of matter in motion; the movements which occur within the range of human observation are sufficient to enable us from them to calculate the paths which they follow when they pass beyond our ken, and the correctness of our calculations is demonstrated when they reappear at the time and place predicted. The chemist recovers on one side of his equation every atom which the other side requires him to account for. The stars in their courses confirm the calculations of the astronomer. Matter is in perpetual course of redistribution, and the same everlasting laws which determine the forms into which it is incessantly being redistributed necessarily determine that those forms shall perpetually improve. This optimistic view of evolution has met with general welcome, but on very different grounds in different cases. Believers in Divine Providence have eagerly greeted it as a startling and irresistible demonstration that their belief in a Providence over-ruling all things for good was true. No suspicion here was possible that the argument had been sophisticated by those with whom the wish was father to the thought. By science the testimony of science could hardly be impeached; and here was science on independent reasonings of its own, starting from purely materialistic ground, compelled by the force of its own arguments to bear witness to the truth which religion had so long proclaimed on the strength of faith alone. To this generation a sign had indeed been given.