Animal Pain In T. S. Lewis The Problem Of Pain

I must admit that I do not fully understand why Lewis chose to include a chapter about animal pain in this book. To me it has no value in his argument about why Man must suffer the problem of pain, and seems to not expand the subject at all. While I find this chapter unnecessary, I cannot honestly say that upon further readings, discussions or thoughts, that my opinion will not change, but that at the present time this chapter seems wholly out of place in his argument.
Lewis closes the book with a chapter on Heaven, affirming that “a book on suffering which says nothing of heaven, is leaving out almost the whole of one side of the account. Scripture and tradition habitually put the joys of heaven into the scale against the sufferings of earth, and no solution of the problem of pain which does not do so can be called a Christian one” (p.148). He contends that nowadays, we are very timid about mentioning heaven, afraid of the ridicule about ‘pie in the sky’ and dreaming of a happy world elsewhere, escaping our duty to make this world a happier place. Lewis postulates, “But either there is "pie in the sky" or
I feel that his arguments would not sway someone who does not believe in the basics of Christianity, such as an atheist. Lewis, instead of answering some questions raised, from an opposite view of his, simply chooses to decline to give voice to these opposing arguments. In my opinion his argument would have benefitted from him expressing his views on these positions, rather than just avoiding them. I found this frustrating, that he quickly dismissed this views, in sections of the book. Again I would like to express that the chapter on animal pain provided, to me, no furtherance of his central argument, but would like to state that I will revisit this section, and do some research on the