In World of Life, Alfred Russel Wallace, Darwin’s co-theorist, directly addressed one of Darwin’s key reasons for rejecting design in nature, in a letter to American supporter Asa Gray:
With respect to the theological view of the question; this is always painful to me.— I am bewildered.— I had no intention to write atheistically. But I own that I cannot see, as plainly as others do, & as I shd wish to do, evidence of design & beneficence on all sides of us. There seems to me too much misery in the world. I cannot persuade myself that a beneficent & omnipotent God would have designedly created the Ichneumonidæ with the express intention of their feeding within the living bodies of caterpillars, or that a cat should play with mice. Not believing this, I see no necessity in the belief that the eye was expressly designed. On the other hand I cannot anyhow be contented to view this wonderful universe & especially the nature of man, & to conclude that everything is the result of brute force. I am inclined to look at everything as resulting from designed laws, with the details, whether good or bad, left to the working out of what we may call chance. Not that this notion at all satisfies me. I feel most deeply that the whole subject is too profound for the human intellect. A dog might as well speculate on the mind of Newton.— Let each man hope & believe what he can.— (Letter 2814 — Darwin, C. R. to Gray, Asa, 22 May [1860])
Now, Darwin was a slippery character, as biographers have acknowledged, and he had been a materialist atheist long before he had any theory of evolution to propose, so his pretense of coming to these conclusions reluctantly was just that – a pretense. (See Flannery on this.)
However, Wallace addresses both examples in The World of Life. With respect to insects, he notes,
There is, of course, a large body of facts which indicate that whole classes of animals, though very highly organized, suffer nothing which can be called pain, as in the insects; and similar facts show us that even the highest warm-blooded animals suffer very much less than we do. (P. 185)
Now, re insects, Wallace is surely right, and I have never been much impressed by Darwin’s example of the Ichneumon wasp laying its larvae in caterpillars. There is little evidence that the caterpillar knows or cares that it simultaneously gorges and is gorged. Whether a given caterpillar pupates or dies is not an instance of any great evil in the world, provided no ecology is upset.
About “the highest, warm-blooded” animals, I am not so sure. However, one source of human suffering that animals don’t have is a “metacognitive” understanding of their condition. That is, the old dog Rover may think, ”I feel sick. I have no appetite, no energy. I just want to sleep all the time.” His people know, “Rover has an inoperable cancer. Sedatives and painkillers for now. Later, we must make a decision …” Rover is forever barred from knowing the nature of his condition, in the human, metacognitive sense, so there are many sources of suffering he simply cannot experience.
With respect to cats, Wallace notes, “It must be remembered that in a state of nature the Carnivora hunt and kill to satisfy hunger, not for amusement; and all conclusions derived from the house-fed cat and mouse are fallacious.” (p. 181)
One might add that the biggest worry for a wild cat or other small carnivore that its catch might be stolen by a bigger animal. Swallowing the prey whole is a common preventive tactic. (The prey may be disgorged later, of course, for offspring – but meanwhile, it is secure down the hatch.)
See Michael A. Flannery’s Alfred Russel Wallace’s Theory of Intelligent Evolution (Erasmus, 2009) for more.