The idea that looks are determined by use.

This principle is used as a conscious design criterion, but it also occurs as a natural tendency.

As an example of the latter, why do Eurasian/American wolves look so much like the (now extinct) Tasmanian wolf? Well, if in two worlds of mammals, a creature takes the role of a wolf, it will evolve to be optimized to play that role, and the design to optimize for 'wolfness' is unique.

Another example: predators evolve to be powerful enough to kill their largest prey, and maintain a balance of power among themselves. (An observation.) This is only to be expected: with less power, something else will evolve to to top of the food chain instead; with excess power, a leaner, more efficient being will evolve and be more competitive. For solitary species, this power is largely a function of the creature's strength and agility, so on top of the food chain you expect large, fast moving creatures with vicious weapons. Some of their prey will 'counterevolve' to become simply too large to catch. And this is what you see everywhere. Aspects of the form of these creatures are "dictated" by their function within the ecosystem.

At the same time, there is a lot of variability there. For instance, in South America, the large predators were giant birds, until the Panama land bridge brought a flood of mammals from the North that completely upset the ecosystem and wiped out the birds instantly.

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