Your cat is territorial, like most cats are.

 Your cat is territorial, like most cats are. A new kitten is an unwelcome intruder in your cat’s mind. Therefore, you should ease their anxiety and introduce them gradually. All their initial encounters should be supervised BY YOU. Please, there’s tons of instructions on this topic here on Quora. Just enter “introducing two cats” in a household in the “search Quora” box (up there on the right side of the screen), and you’ll find all answers you need and all tricks you can use to speed up the process. But with cats, it always takes (at least some) time. Do not neglect your cat (that is the first-comer) on the account of the new kitten. With cats it’s the “first come - first served” rule. Do not deprive your “up to recently only cat” of any of his/her privileges, the new kitten -as adorable as it surely is- must adapt and find its place in a new home. And respect the authority of those who were there before him/her. Cats usually do that without too much help or interference from their humans. Just follow the rules, pay attention and intervene only when it’s absolutely necessary.

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