New collaboration between biologists and mathematicians analyzes the hidden complexity of animals and plants, offering new insights into the behavior of entire organisms, their interactions, and the impacts on biological diversity in the planet's ecological web. Mathematics provides new and sometimes impressive perspectives on evolution and inheritance and population methods on time scales ranging from millions to hundreds of years, as well as what is happening now. Ian Stewart, with his characteristic clarity and entertaining style, explores these and many other related themes, including the role of genes in controlling behavior and the nature of life. He demonstrates how mathematicians and biologists manage to tackle some of the most challenging scientific problems humanity has faced and where their research is heading.