Ernst Mayr No two individuals in sexually reproducing populations are the same, no two populations of the same species, no two species, no two higher taxa, nor any associations, and so ad infinitum. The Growth of Biological Thought, 1982.
Evolutionary biologist, philosopher, and historian of biology. Also the author of Populations, Species, and Evolution, Animals Species and Evolution, andEvolutionary Synthesis: Perspectives on the Unification of Biology. Wherever we look, we find uniqueness, and uniqueness spells diversity. Ibid. The basic, direct evidence for evolutionary change is twofold: for horizontal evolution the nonconstancy of species as revealed by geographical researches, and for vertical evolution the fossil record, as revealed by geological researches. Ibid. The areas which had already supplied Darwin with the most telling evidence for evolution - paleontology, classification, biogeography, morphology, and embryology - have continued to supply the most convincing proofs of evolution up to modern times. Almost, the only recent, but in fact highly important, addition is molecular biology. Ibid. The genetic material (DNA) is completely constant ("hard") from generation to generation, except for a very occasional (about one in 100,000) "mutation" (that is, an error in replication). Ibid.