This digital document is an article from Macmillan Reference USA Science Library: Biology, brought to you by Gale®, a part of Cengage Learning, a world leader in e-research and educational publishing for libraries, schools and businesses. The length of the article is 268 words. The article is delivered in HTML format and is available in your Amazon.com Digital Locker immediately after purchase. You can view it with any web browser. A comprehensive collection of articles on all aspects of biological science, including its history and issues. Includes articles on biology-related careers and important individuals, as well as topics from active transport to zoology research.