It is interesting to note that most contemporaries associate theories of evolution with Darwin, when there were others who were talking about evolution and natural selection before him. This is especially important to point for a discussion of "In Memoriam" because Darwin's On the Origin of Species was actually published nine years after Tennyson's poem. Before Darwin's book the more prevalent texts on theories of evolution--both human and the world--were Charles Lyell's Principles of Geology and Robert Chambers Vestiges of the Natural History of Creation. Lyell's book questioned common Christian beliefs of a time of catastrophic change for the Earth, and insetad proposed that the " formation of Earth's crust took place through countless small changes occurring over vast periods of time, all according to known natural laws" ("Charles"). While many of Lyell's suggestions have now been deemed entirely wrong, we can examine the impact that this groundbreaking text would have had on the people of Victorian London, more specifially on Tennyson. Tennyson's poem while sometimes seen as merely a poem of a crisis of faith. However, it is also about Tennyson's crisis of doubt. Lyell's text, while challenging beliefs, also attempts to reconcile science and religion. Tennyson's poem is also a narrative that looks at his crisis of faith, but because it was still so important to him, his poem also becomes one where we can the "crisis of faith and the crisis of doubt are emphatically part of the same crisis" (Davis). Tennyson's crisis and reconciliation can be clearly seen in Obit CXXIII where he says, "I found Him not in world or sun/Or eagle's wing, or insect's eye;/Nor thro' the questions men may try,/The petty cobwebs have spun:/If e'er faith had fall'n asleep . . ./like a man in wrath the heart/Stood up and asnwer'd, 'I have felt.'" The basis of his faith can longer be built from nature, bur rather it must come from an internal source. 

Davis, Phillip M. “Crisis of Doubt: Honest Faith in Nineteeth-Century England.” The Catholic Historical Review, vol. 94, no. 2, 2008, pg. 397-398. https://search-proquest-com.ezproxy.greenriver.edu/pqrl/docview/20004051...
“Charles Lyell: Principles of Geology.” Evolution. PBS, 2001. https://www.pbs.org/wgbh/evolution/library/02/4/l_024_01.html