…during which time the basic design (the morphology) of tree squirrels has remained unchanged. That’s because the design works so well that there is no need for evolution to change it.
Squirrels have ankles that rotate 180 degrees, allowing them to climb downward. Their tails provide both shade and cooling as well as insulation and protection from cold. And although they fall more often than we think, they always flip right side up, assume a glide posture, usually land softly, and are almost never injured.
And that ain’t the half of it.