The third edition of this comprehensive texbook has been written to reflect developments in the specifications for GSCE Science; the biology section of GCSE Science Double Award and equivalent examinations. Amendments have been made to the text and illustrations. In particular: there is additional information on topics such as clinical trials, B and T lymphocytes, infertility, performance-enhancing hormones, fluoridation of water supplies, impact of hunting and over-fishing, global warming, biofuels and global travel and disease; an additional chapter on applied genetics draws together and expands the information on selective breeding, genetic engineering, cloning, genetic fingerprinting and the Human Genome Project; an additional chapter on conservation covers extinction, conservation of species, conservation of genes; conservation of habitats and conservation of resources; a revised chapter on ecosystems includes information on adaptations and factors which affect communities; examination questions have been updated using past papers; there is an additional chapter on the development of biological ideas and the principal contributions to these ideas from Galen, to Watson and Crick; and a final chapter on observation and experiment which looks at the type of questions that can be investigated scientifically and the nature of the evidence.