Darwin made many observations which later led to his acceptance of evolution. His observations on the Galapagos islands were especially significant. There he saw the evidence of change in isolated populations. Turtles, mocking birds and finches were unique to each island even though they were obviously derived from the same mainland species. He saw first hand that environments different from the mainland had resulted in modified turtles and birds. However, the idea of evolution by natural selection did not mature until after his return. The ornithologist, John Gould, pointed out to Darwin that the mockingbirds which Darwin had collected from different Galapagos Islands were so distinct from one island to another as to represent different species. This revelation seems to have led Darwin to doubt the fixity of species and to set about gathering evidence on the "transmutation of species". The theory of Natural Selection crystallized on September 28, 1838, 2 years after he returned from the voyage of the Beagle. He recounts in his autobiography that, " I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved and unfavourable ones destroyed.".
