Walter Garstang: a retrospective.

Although, Walter Garstang died over 60 years ago, his work is still cited--sometimes praised, but sometimes belittled. On the negative side, he often appropriated ideas of others without attribution, ignored earlier studies conflicting with his theories, and clung to notions like inheritance of acquired characters, progressive evolution, and saltation after many of his contemporaries were advancing toward the modern synthesis. Moreover, his evolutionary scenarios--especially his derivation of vertebrates from a sessile ascidian--have not been well supported by recent work in developmental genetics and molecular phylogenetics. On the positive side, Garstang firmly established several points of view that remain useful in the age of evolutionary development (evo-devo). He popularized the valid idea that adaptive changes in larvae combined with shifts in developmental timing (heterochrony) could radically change adult morphology and provide an escape from overspecialization. Moreover, his re-statement of the biogenetic law is now widely accepted: namely, that recapitulation results when characters at one stage of development are required for the correct formation of other characters at subsequent stages (his stepping stone model). In other words, ontogeny creates phylogeny because some developmental features are constraints, favoring particular evolutionary outcomes while excluding others. This viewpoint is a useful basis for advancing concepts of homology and for comparing the phylogeny of ontogenies across a series of animals to ascertain the timing and the nature of the underlying ontogenetic changes.