The Evolution of
Complexity, by Means of Natural Selection
The cellular slime molds are a very peculiar class of creatures, neither animal nor plant, which spend most of their life as separate cells, roaming through the soils and leaf-litter of forests. When their food grows scarce, they agglomerate into a single ``colonial'' organism, rather resembling an amoeba, which feeds for a while before fixing itself in place and growing a stalk, with so-called ``fruiting bodies,'' from which spores are released --- the only phase of the life-cycle in which there is any specialization of cells. Bonner discusses this in Evolution of Complexity, and more fully in his The Cellular Slime Molds.