Contemplate all this work of Time,
The giant labouring in his youth;
Nor dream of human love and truth,
As dying Nature's earth and lime;
But trust that those we call the dead
Are breathers of an ampler day
For ever nobler ends. They say,
The solid earth whereon we tread
In tracts of fluent heat began,
And grew to seeming--random forms
The seeming prey of cyclic storms
Till at the last arose the man; (. . .)
--from In Memoriam by Alred, Lord Tennyson
In this particular segment of In Memoriam, Lord Alfred Tennyson incorporates imagery calling back to the latest discoveries on geologic developments put forward in Lyell's Principles of Geology. The allusion is useful as an example of the important theoretical concepts in Barthes, Bahktin, and Derrida. Lyell's book marked one of many major scientific treatises of the Victorian age, which had great impact on the conventional worldviews of that time.
Barthes would find this piece interesting because it clearly demonstrates the operation of an omnipresent cultural "code." Tennyson's text draws from Lyell's text (a work from an entirely different discourse) and uses it for new purposes--namely, in contemplating the purpose of human existence.
Derrida might add that this poem is a good example of a trace. Indeed, In Memoriam owes its materialization to Lyell to a large extent. The despair that the speaker experiences is partly inspired by the struggle to reconcile the plight of existence in a world impartial to all life forms, as Lyell's book suggested. Therefore, In Memoriam is indebted to another text, another set of signs, for both its own production and interpretation. This is quite in keeping with what the concept of trace avers--that meaning is born of other meanings.
Bahktin would analyze this reference differently, most likely by putting an ideological slant on the text. The poem can be seen to mark a classic illustration of how heteroglossia gives people the opportunity to originate an internally persuasive ideology out of the external ideologies circulating in their culture. In Memoriam visibly contains a dialogue between two competing ideologies (among others): science and religion. To elaborate, Lyell's work signaled a challenge to the traditional religious ideology of the Victorian age by questioning mankind's eminence over other species and closeness to divinity. Tennyson, undoubtedly like many others in his day, was incapable of ignoring one over the other. But at a certain point in the poem, he was able to negotiate this tension by shaping a personal way of looking at the world that paid respects to both viewpoints, keeping the contradictory assertions of each separate yet intact.