Canis familiaris is the least uniform species on the planet.

No doubt some questions are more consequential than others but fortunately that has no bearing on whether they are interesting questions. From Do Dogs Know Other Dogs Are Dogs? by Julie Hecht.

Does a dog know, merely by sight, that an approaching being is a fellow dog? Before you answer, remember this: Canis familiaris is the least uniform species on the planet. Members of this species come in a wide range of body shapes and sizes from itty bitty teeny weeny to absolutely ginormos. Adult members of this species appear as tight little packages, huge weightlifters, lean ballerinas, elongated hotdogs and everything in between.

Does a Pug look at an Afghan Hound and say to themselves, "Hello, dog!" or does a Pug look at an Afghan Hound and say, "WHAT IN THE WORLD ARE YOU?" and only after olfactory investigation (smelling) does the Pug realize, “Oh my goodness. How silly of me. You're a dog. Sorry for the confusion my large, long-snouted compatriot."

A number of researchers have essentially wondered what Pugs think of Afghan Hounds. Are dogs able to identify other dogs solely by appearance, they wondered? If olfactory cues are taken out of the equation, would a dog still know another dog when he sees one?

Is that really true that "Canis familiaris is the least uniform species on the planet"? I can imagine it is but I have never seen that claim before.