Before 1846, Gray had been firmly opposed to the idea of transmutation of species, in which simpler forms naturally become more complex over time, including via hybridization. By the early 1850s, Gray had clearly defined his concept that the species is the basic unit of taxonomy. This was partly the result of the 1831–1836 voyage during which Darwin discovered the differentiation of species among the various Galápagos Islands. Local geography could produce variances, like in the Galápagos and Hawaii, which Gray did not get to study in depth as he had wanted. Gray was insistent that a genetic connection must exist between all members of a species, that like begat like. This concept was critical to Darwin's theories. Gray was also strongly opposed to the ideas of hybridization within one generation and special creation in the sense of its not allowing for evolution.