Evolution seems to close the heart to some of the plainest spiritual truths while it opens the mind to the wildest guesses advanced in the name of science.

In this line Bryan contrasts what he presents as “plain” religious certainties with what he characterizes as speculative scientific theorizing. The rhetoric reflects his broader anti-evolution stance in the 1910s–1920s, when he argued that Darwinian evolution was not merely a biological hypothesis but a worldview that could erode faith and moral responsibility. By saying evolution “closes the heart,” he frames acceptance of evolution as a spiritual and ethical disposition, not just an intellectual conclusion, while “wildest guesses” casts scientific inference as reckless conjecture. The quote thus functions as a polemical warning: scientific authority, in Bryan’s view, can be misused to displace spiritual truths with materialist assumptions.