From Edgebee Wiki
In Critter Forge there are 20 species. They are devided in four tiers with five critters each. The affinity to a tier determines certain characteristics of your critter as shown in the table below.
|Credits per Level on Retirement||5||10||15||20|
*) Gestation time in this table shows how much each tier contributes to the gestation time of an egg. For example, if you mix a tier I and a tier III samples, an egg will hatch in 12 hours.