Explored in this book are the effect of environmental changes on most animals (especially poiilothermic animals) at two levels, the proximate process, where environmental changes affect their development, and the ultimate process level, where natural selection acts on the changes undergone during the proximate process. It reexamines neo-Lamarckism in light of 20th century biology and shows it as a major pattern of evolution. The book also focuses on how environmental factors influence endocrine balances that in turn affect gene regulation.