But it is also certain that, before the deposition of the chalk, a vastly longer period had elapsed;throughout which it is easy to follow the traces of the same process of ceaseless modification and of the internecine struggle for existence of living things;and that even when we can get no further back, it is not because there is any reason to think we have reached the beginning, but because the trail of the most ancient life remains hidden, or has become obliterated.
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