The chapters presented in this book were written by Thomas Hunt Morgan, an American evolutionary biologist, geneticist, embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role that the chromosome plays in heredity. Here, he lays out his ideas on the relations between the organism and its environment and whether Darwinian ideas are enough to explain some of the phenomena he observed during his studies, especially those of regeneration.