Christopher Columbus crossed the Atlantic in 1492 and opened a period of extensive exchange between the Old and New Worlds. His greatest impact on the New World has been the one to which the least attention has been paid: his biological impact. For millions of years the biotas of the Old and New Worlds developed independently, divergently. The demographic takeover of North America by large numbers of European humans is but a part of the general invasion of that continent by a portable collection of lifeforms that the explorers, colonists, indentured servants, and slaves brought with them, intentionally or unintentionally. Many plants, especially weeds, took root in the New World and flourished, often crowding out the native flora. Very few plants traveled east to Europe and achieved naturalization there. Native Americans domesticated only the dog and the turkey. Domesticated horses, cattle, pigs, goats, sheep, asses, chickens, cats, and more were brought from the Old World. The efficiency and speed with which they altered their new environment made them superior to any machine ever built. Old world pathogens were the Native Americans' worst enemy. Smallpox, measles, chickenpox, influenza, malaria, yellow fever, diptheria, whooping cough, typhoid fever, scarlet fever, amebic dysentery, and other diseases spread in epidemics that decimated the population. The success of these imported biotas is due to the fact that: (1) they were organisms that thrive in environments of disruption; (2) they came from highly competitive ecosystems and so were hardy; (3) they included many small and invasive organisms; and (4) they were mutually supportive. A 6-item bibliography is included. (JB)