Sumatra followed the rule of the survival of the fittest. A fit existence, in essence, adapts to the challenges imposed upon it, whether it be environmental, societal, resource, and power.
If it continues to survive and create more of its kind over time, causing the race's population to either expand steadily or ensure it gets maintained consistently through the test of time, then the race is termed 'Fit.'
There were races that took advantage of this survival of the fittest to push themselves to the extreme, until they adapt to the extremity. This is how Mutated Pranic Beasts appear.
Of course, the extremity they subject themselves towards should be on a path that they could tread upon.
For example, a fresh water fish could try to adapt to salt water. It'll have a small chance of success. However, if it tried to adapt to lava, that was pure foolishness. It had no means or genetic traits that could morph in any means whatsoever to reach the level necessary to live in lava.
