Genetic variation and selection may seem adequate to account for evolutionary adaptation and improvement of single-cell organisms, but they are hardly sufficient to account for the evolution of complex organisms that comprise whole societies of cells that are assembled and reassembled in each succeeding generation and whose genomes have undergone vast expansion.
Butler's epigram that a hen is an egg's way of making another egg can today be viewed as a comment not on the general question of evolution but on the particular question of metazoan evolution. It seems that the biologic novelty that distinguishes metazoa is obviously cellular differentiation, defined as the capacity of a cell to commit itself to a subset of its genes, subserving only a subset of functions necessary for life.
The machinery of differentiation must be a part of metazoan cells and it must involve genes, but the facts of cellular continuity and genetic continuity give little clue to the nature of that machinery. So with that in mind, we might begin discussing that indissoluble trinity: ontogeny, phylogeny, and evolution.
Ontogeny connotes the generation of sets and subsets of cells from the zygote. Ontogeny doesn't end with either the formation of the embryo or completion of the adult organism. Lower vertebrates can replace a