Charles Darwin began his first notebook on the transmutation of species in 1837. He opened the notebook, usually called Notebook B, in July of that year after returning from the voyage of HMS Beagle and examining the specimens he had collected.
The notebook marked a major shift from scattered observations to a sustained investigation of how species might change. Darwin filled its pages with ideas about variation, inheritance, geographical distribution, breeding, fossils, and the relationship between living organisms. He was not yet presenting a finished theory; the notes show a scientist testing possibilities and recording questions.
Darwin's thinking had begun moving in this direction earlier. In March 1837, after naturalist John Gould examined his Galápagos bird specimens and identified distinct species, Darwin became increasingly interested in the stability and transformation of species. His Red Notebook contains some of his earliest explicit thoughts on transmutation.
Notebook B was the first in a series of transmutation notebooks. Darwin later developed the concept of natural selection in Notebook D, and eventually published the mature theory in On the Origin of Species in 1859.