Classification by physical appearance doesn't work. There are many varieties of dogs that look very different from each other. These different varieties of dogs are all the same species. There are many birds that look so much alike that even members of the Audubon society have trouble telling them apart, but they are different species. You can't tell if two critters are the same species just by looking at them.
Appearance is too subjective.
Reasonable people might disagree as to whether or not two critters look enough alike to be the same species. The higher categories are abstract groupings of species. A species is composed of organisms that resemble one another in many important characteristics. Moreover, in organisms that have sexual reproduction, a species is made up of interbreeding populations that, ideally, cannot produce fertile offspring with members of any other species. This is where there is a chain of intergrading subspecies forming a looping or overlapping circle whose terminal links, although inhabiting the same geographical region, do not interbreed even though they are connected by a complete chain of interbreeding populations.