What would I need to focus on to write an essay on the impact of Darwinism on the Victorian literature?
It's unlikely that in a single essay you could cover all the different ways in which Darwin influenced several thousand works of Victorian literature. Instead, you need to select one or more works that directly respond to Darwin. For example, Tennyson's In Memoriam was written to struggle with the problem of how we respond to death in light of Darwin; at first he suggests that while the individual may die, the species or type continues, but then he reflects:
So careful of the type?’ but no.
From scarped cliff and quarried stone
She [nature] cries, `A thousand types are gone:
I care for nothing, all shall go.
H. G. Wells' Island of Dr. Moreau suggests that human attempts to intervene in evolution could result in monstrosities; like Tennyson, he sees the force of nature as far less benign than did Romantics such as Wordsworth who saw Nature as an external representation of divine benevolence.
The main element of a successful literary essay is to narrow your focus sufficiently so enable you to deal with your topic thoroughly.