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The natural world today contains a vast array of man-made chemicals, many of them with biological potency in quantities scarcely measurable tow decades ago. Wild animals encounter these chemicals in multifold combination, but man's knowledge of their effects on wild species progresses haltingly. While research focused on DDT, dieldrin, and related chemicals that were obvious suspects for cause of environmental problems, other compounds, some unidentified, spread and increased.
Polychlorinated biphenyls (PCB's) constitute one such group. PCB's are products of modern industrial chemistry that have escaped to the environment by routes still largely unknown. They have become ubiquitous in the world ecosystem in quantities similar to those of DDE. Their presence has caused concern and stimulated research to evaluate their role in the biosphere.