God help us.

For months after we brought home our beagle, Sasha, to our two previously installed cats, I would look at the photos on the Internet of cats and beagles snoozing together or grooming each other and conclude these images were either computer-generated or posed with animals freshly returned from the taxidermist.

Things couldn't have been going worse. Who knew our obese, somnolent cats could have been instructors for Delta Force? They stalked Sasha in tandem, and when she tried to flee, the cats flanked her, maneuvered her into a corner, and slashed at her ears. Sasha, not a strategic thinker, would then wait until they were asleep and rush at them barking. The aroused cats then slashed some more.

This resulted in a three-species epidemic of stress-induced colitis. I found an over-the-counter treatment for myself, but my veterinarian got out the pad and gave me a prescription for liquid Prozac for Sasha, Biscuit, and Goldie.

No wonder half the population is on the stuff. A few weeks after the cats started slurping Prozac, I noticed that when Sasha (who had to drop her treatment because of explosive diarrhea) tried her usual attack gambit, the cats, instead of trying to ride her bareback, merely waved her away, like the cool kids amused by the class dork.

In the movie Ghostbusters the apocalypse is envisioned as, "Rivers and seas boiling! … The dead rising from the grave! ...Human sacrifice! Dogs and cats living together!" Today about 20 percent of pet-owning households are experiencing this doomsday scenario. It looked as if Provo, Utah, had put a stop to such madness last year, when a story made national headlines about the ordinance banning citizens from owning both cats and dogs. It turned out to be a big misunderstanding. A badly worded law limited dog and cat lovers to two pets but made it sound as if the owners had to choose species. The law was amended and now the people of Provo can have a total of four dogs and cats at any ratio they like.

There is even a genre of entertainment devoted to mixed-species living. Take the comic strips Garfield and Get Fuzzy,which both feature a young man, his cynically manipulative cat, and amiable dunce of a dog. The unwatchable movie Cats & Dogs takes the "cat is cunning, dog is dodo" approach even further in its portrayal of evil cats versus a sweetly innocent beagle. Harvard President Larry Summers, when he's through with his gender re-education, should form a task force on species stereotyping.

In consulting behaviorists and friends with pet-diversity issues, I discovered my experience falls somewhere in the middle. I didn't mind that my pets would never be models for interspecies love, as long as my cats weren't ending up on a slab in the Special Felines Unit. Stephen Zawistowski, senior vice president at the ASPCA, once had a Doberman and a cat who were so in tune the dog allowed the cat to rub around her legs, scent marking her. Now he has a beagle and a cat, and he finds they are like having a teenage boy and girl in the same house. "They have different forms of existence," he says. "They pass each other."

Zawistowski says one big problem that occurs between the two species is that they misinterpret each other—the way an American's thumbs-up gesture can be the equivalent of giving the finger in the Middle East. For example, a dog rolling on its back is either signaling submission or that it's ready to play. A cat on its back is in full defensive mode, says Zawistowski, "ready to slash with all four feet and use its teeth."

There is also a Punch-and-Judy aspect to many dog-and-cat encounters. A dog indicates it wants to play with a cat by literally getting in its face; the cat responds with a swat to the snout. If you've ever sat through a Punch-and-Judy show, it feelslike it lasts for years, but these dog-and-cat shows often actuallydo. That the dog never stops trying proved to be a tonic for the cat of Gene Carter, a retired professor of finance in Washington. His 17-year-old cat, Amos, had a stroke that paralyzed a back leg. The younger beagle, Spooky, continued to badger the ailing cat so much that Amos' efforts to escape from the dog constituted physical therapy. Amos regained the use of his limb and lived to run away from Spooky for another three years.

Sometimes the dog acts brutish to the cat all day, then insists on cuddling up with it at night—think of this as Stanley Kowalski syndrome. My friend Cindy says often she'll come home and find the Lhasa apso, Cinnamon, and the cat, Rosie, sleepingnext to each other. But once they're awake, Cinnamon jealously guards the family, barking at Rosie if she tries to get near them. Rosie then banishes herself to the bathroom, where she waits on the mat for a pat from passing family members. Cindy has unwittingly followed an important dictate the experts say is crucial in a cat-and-dog household: Each species must have the equivalent of a room of its own. For cats this can be counters or high furniture, for the dog a crate. An indoor cat must also have a litter box that's out of the dog's reach, so the dog can't use it as a source of breath mints.