"The Variation of Animals and Plants Under Domestication, Vol. I." by Charles Darwin is a two-volume work published in January 1868. This comprehensive study presents detailed observations on domesticated animals and plants, drawing from Darwin's years of breeding experiments and correspondence with fellow naturalists. The work also introduces his controversial theory of heredity called pangenesis. Written over four years following "On the Origin of Species," the book represents Darwin's attempt to provide the detailed evidence and facts supporting his evolutionary conclusions, though he found the writing process exhausting.