The essay I have in mind comes from God in the Dock, and is titled "The Pains of Animals." It is actually more than an essay; it presents an 'inquiry'--from C.E.M. Joad--regarding chapter nine of Lewis' The Problem of Pain followed by the latter's reply.
I find a couple of Lewis' quotes worth sharing. In attempting to clarify his earlier claims, he summarizes the least speculative part of his original treatment...
The data that God has given us enable us in some degree to understand human pain. We lack such data about beasts. We know neither what they are nor why they are. All that we can say for certain is that if God is good (and I think that we have grounds for saying that He is) then the appearance of divine cruelty in the animal world must be a false appearance. What the reality behind the false appearance may be we can only guess.As an ecologist (and as I have perhaps hinted already), I take issue with the widespread perception that the animal world is "cruel." That is, I would heartily affirm that the appearance of cruelty is a false one, or (better yet) a subjective perspective. But my main point here is to affirm Lewis in his willingness to remain agnostic about the issue, to give God the benefit of the doubt where his (Lewis') knowledge remains imperfect.
Lewis himself arrives (a bit further on) at the recognition of the subjectivity involved here. This, too, is perceptive, and constitutes an important counterpoint to the argument against God made by appealing to animal death.
If I regard this pity and indignation [at the suffering in the insect world] simply as subjective experiences of my own with no validity beyond their strength at the moment (which next moment will change), I can hardly use them as standards whereby to arraign the creation. On the contrary, they become strong as arguments against God just in so far as I take them to be transcendent illumination to which creation must conform or be condemned. They are arguments against God only if they are themselves the voice of God... That the mere contingent Joad or Lewis, born in an era of secure and liberal civilization and imbibing from it certain humanitarian sentiments, should happen to be offended by suffering--what is that to the purpose? How will one base an argument for or against God on such an historical accident?In other words, one very reasonable response to the perceived problem of animal suffering is that God's workings in the planet's ecology are not what some modern people perceive them to be. That is, perhaps God has good reasons for creating a world that involves animal death, and our perception of such death as bad or evil is wrong. As I see it, the types of evidence that lead to this conclusion include historical, Scriptural, and ecological. That being so, both those who appeal to animal death in arguing against God and those who build strange Bible interpretations around a felt need to absolve God of responsibility for animal death are sadly misguided.