According to Twain why is the cat innocent for doing the same thing a person might do

That the human race is of one distinct species. It exhibits slight variations (in color, stature, mental caliber, and so on) due to climate, environment, and so forth; but it is a species by itself, and not to be confounded with any other.

That the quadrupeds are a distinct family, also. This family exhibits variations - in color, size, food preferences, and so on; but it is a family by itself.

That the other families - the birds, the fishes, the insects, the reptiles, etc. - are more or less distinct, also. They are in the procession. They are links in the chain which stretches down from the higher animals to man at the bottom.