Why was the Secret Garden full of cats and not a different animal?

I think that cats represent a mysterious sort of animal. They don't lavish themselves over man rather than pick and chose who they want to keep company with. Many "dog people" feel cats are arrogant and aloof. For this reason it makes sense that cats should have their own world. Why should cats share the dangerous dirty streets of man? I think the birds come in as an extension of the natural elements of things. Cats would of course permit birds in their world; cats love birds.