It’s the solitary ones who are most vulnerable — those of us who live by ourselves and have time, probably too much time, to think. It happens gradually, imperceptibly, like temperature rising or water seeping, and one day you find yourself noticing new lines, say, in his facial markings. You notice the way he greets you, nuzzling your outstretched finger, then sliding his mouth along your fingertip to the corner of his jaw. You notice the whites of his eyes as he watches you continuously, not out of wariness, but out of a gentle, calm trust we humans would call love. You notice the nuance in the way he moves, the subtle pauses and postures that express his own personality and distinguish him from other cats — and you hear the particular timbre of his voice and know intuitively with a crawling of the nape when he’s threatened by another cat out in the wilds beyond the door. You realize at some point that his movements and gestures are a language, his tail wrapping gently around your leg, or his head pressing deliberately into your hand, or his mouth opening in a wide, fang-bearing yawn of greeting as you walk into the room. The way he stretches forward and claws the rug, the little crook in the end of his tail, the unique tufting of his belly fur — these quiet, introspective revelations are the gift of the cat to the solitary person, for the cat is a creature with whom you share solitude. A human being, on the other hand, is a creature with whom solitude is generally a failed relationship. With one the essence of success is communion. With the other it is communication. One depends on spoken language and rational intellect, the other on the language of gesture and intuition, and whereas communion with an animal is considered inferior to communication with a human being, the truth is, the need for companionship of any sort is a human-species trait, and in the absence of a human companion, the mind grows like a vine around any living thing. The first time your mind grows around a cat, you do not realize you have fallen in love.

Communion with a cat takes time to mature, and it is irreversible. Those who find it are forever altered and cannot go back to the way they once were because the mind, the soul, the eye of self, arises from the physical substance of the brain, and that substance has been altered. . . .

A physical mechanism — a neuronal machine — is gradually assembled to service the relationship, and details accumulate in the mind as more neurons, more synaptic connections are dedicated to your companion. Those who work at home and live the single life can easily spend 80 to 90 percent of existence with their animal comrades, which means that a very large mechanism indeed must be constructed.

You don’t realize how pervasive this mechanism has become until your companion is taken ill; then the world cracks and crumbles around you. His suffering becomes your suffering. When he lies in pain and silence, you immediately grow depressed. If he shows the slightest sign of recovery, the sun shines into your soul and your spirits soar euphoric. In other words, the health of your companion controls your moods as if your nerves were linked directly together. You are fully aware of this influence; you just cannot control it.

And when your companion dies, the pain is almost unbearable. The longer and the deeper you love him, the greater the price in grief. It’s as if part of your self has been amputated without anesthetic, which it probably has — literally — because the machinery needed to generate the miraculous subtlety and nuance you experience with your loved one is, in one ineffable instant, rendered moot. It has no more reason for being. . . .

Meanwhile the memory mind continues to operate as if your friend still lived, projecting images in all the places he loved to be, and you see him everywhere, lying on the bed, sleeping on your desk, jumping over the wall and walking gracefully to greet you on your return home. The fact is, those we grow to love continue to live in the synapses and molecules of memory, and as long as we exist, so they exist as part of the brain. That is what happens when anyone loves anyone, or anything. It doesn’t matter to the neurons deep in the brain whether those whom you loved were human or animal.

Excerpted from A Cat Named Darwin by William Jordan. Copyright © 2002 by William Jordan. Reprinted by permission of Houghton Mifflin Harcourt Publishing Company. All rights reserved.