Nikratio

Hi, I've just started playing Geneforge Mutagen and I'm a bit confused by the background story. Seemingly it takes permanent essence to make a creation, and the shaper can get the essence back by absorbing a creation. In that case, why is the island full of "leftover" creations? Doesn't that mean that a large number of shapers permanently gave up part of their essence, when they could have trivially absorbed at least the non-rogue creations? Why would they do that? Or is this part of the storyline and explained later?