This could be subtitled: versions of a not-memorable ditty…or, ticketing in the old days….
A dog is a dog and a cat is a dog, and a squirrel in a cage is a parrot, but a tortoise, he’s a hinsect, so he rides free.
I heard this long ago, and vaguely remember it referred to ticket costs when riding? what, a train? a bus?, but nothing more.
Now, buried in the on-line correspondence in the New York Review of Books is this, attributed to Freeman Dyson:
When I was a boy in England long ago, people who traveled on trains with dogs had to pay for a dog ticket. The question arose whether I needed to buy a dog ticket when I was traveling with a tortoise. The conductor on the train gave me the answer: “Cats is dogs and rabbits is dogs but tortoises is insects and travel free according.�?
And this, from a 1869 Punch cartoon caption of a railway porter advising a woman traveling with her no-doubt beloved animals:
“STATION MASTER SAY, MUM, AS CATS IS ‘DOGS,’ AND RABBITS IS ‘DOGS,’ AND SO’S PARROTS; BUT THIS ’ERE ‘TORTIS’ IS A INSECT, SO THERE AIN’T NO CHARGE FOR IT!�?
…both from Nicholas Humphrey, resident of Cambridge, England.
Well, this is as far as I can go; maybe someone knows more of the story, or how this came into folk memory (a quick Google turns up nothing)…. Certainly, classification has deep human roots….