I have to disagree with that on a minor point. In the beginning, Garfield was strictly a quadruped. In an interview on KSFO with WND columnist Barbara Simpson, Jim Davis said that it was none other than Charles Schulz who convinced him to expand Garfield's horizons by having him walk upright like Snoopy does.
If you can, check out the very, very early Peanuts strips, when the main characters were Charlie Brown, Shermy, Patty (not Peppermint Patty) and Lucy (Linus and Schroeder were born into the strip later). Snoopy was strictly a dog, with no vivid imagination causing him to fantasize about being a World War I pilot or an astronaut.
As for Calvin & Hobbes: reading an interview with Bill Watterson is a maddening experience. He took the strip seriously, which is good, but he intellectualized the characters in his own mind to an extent that it makes you wonder if he is sane.
In Charlie Brown, Snoopy and Me, Shultz explained that Snoopy was based on a dog he'd owned as a child, as was the Charlie Brown character (Shultz's father was even a barber, like Charlie Brown's). He said Snoopy started out as a dog, but he slowly started reacting to the children around him, and just gradually grew into a major character. He also said he had to work hard to keep Snoopy from taking over the entire strip, especially after Woodstock came along to be a side kick that could interact with him more readily.
What I was saying was, that without Snoopy and Garfield, there would not have been a Hobbes. Certainly there were other influences. Winnie the Pooh, for example, was another character that existed as a toy in real life, but was alive in the eyes of his owner. Hobbes, though, traces his roots, I believe more directly through Snoopy. Prior to Snoopy, comic strip animals were pretty much, well, animals. Ruff was never anything but a dog. Dittoes for Barfy (Family Circus), Pluto (Mickey Mouse), Daisy (Dagwood and Blondie) and most other cartoon pets. They were primarily animals. After Snoopy, thinking animals appeared throughout comics. Garfield had an edge of sarcasm that Snoopy never had, and I think this edge influenced Watterston in the creation of Hobbes.
I used to teach cartooning classes (non-credit) at the University of Texas, so I've thought about this a lot more than is probably healthy. I'm surprised at the number of people who have reacted to my post.
I do consider the tie between Hobbes and Snoopy to be relatively minor, and am convinced that the two major influences on the strip were "The Secret Life of Walter Mitty" and the Chuck Jones series of cartoons about the daydreaming boy.