Holland starts by addressing the fundamental question of what defines an animal. This section highlights the incredible diversity within the animal kingdom, emphasizing that the definition is not as simple as it appears. By exploring various characteristics and classifications, readers gain a deeper understanding of the vast array of life forms that fall under the category of animals.

One of the central themes of the book is the evolutionary relationships among different animal groups. Holland uses molecular data to shed light on how various species are related and how they have evolved over time. This phylogenetic approach is crucial for modern zoology and provides readers with a clearer picture of the interconnectedness of life on Earth.

The book delves into the biology of each major group of animals, offering insights into their unique characteristics and behaviors. This section is particularly valuable for students and enthusiasts, as it provides a concise yet thorough overview of the biological traits that define each group. By understanding these traits, readers can appreciate the complexity and beauty of the animal kingdom.