Chapter ten tied together the major ideas in the book to provide a large scale summary of animal evolution and the mechanisms behind it. The largest tie is in between the metamorphosis of whales and early tetrapods. Both lines began with a small group of animals exploring the borders of a new ecosystem ripe for colonization. The exaptations of lungs in tetrapods and long tails in mesonychids became essential for their survival in a new environment. Stepping back to view the whole process, we see that drastic changes in the environment open up and close routes of life and change. The ability of an organism to adapt to those changes is ruled by their genes and how they develop, in other words animals are “constrained by their history.” To fully understand the transitions groups undergo the history of that group’s branch must be considered. On a similar note, Zimmer brings back the characters from the first chapter of the book, Richard Owen and Charles Darwin, to symmetrically complete it. Transitional fossils build on Darwin’s original ideas to tell the story individual lineages and the overall story of macroevolution.