Teratology is most simply defined as "the study of environmental agents which disturb development". A more comprehensive definition is "the study of causes, mechanisms, and manifestations of developmental deviation, structural or functional." Teratology has several important purposes. Obviously, the most important is to protect future generations by identifying environmental agents that cause malformations. In this sphere, ophthalmologists are generally not the primary detectors. One notable exception is when Gregg, in 1941, recognised that an increase in congenital cataracts was due to a viral agent and brought rubella embryopathy to the attention of the medical community. Another pertinent reason for teratology is that it gives us insights into normal development. As Harvey said with such flair, "nature is nowhere accustomed more openly to display her secret mysteries than in cases where she shows tracings of her workings apart from the beaten path".