The study of identical twins has helped to elucidate many problems bearing on the influence of heredity and environment. When one reads in the literature the reports of case after case exemplifying the bond between identical twins in health and their morbid sympathy in disease, one can readily grasp the importance of inborn factors on mind and body.
DEFINITIONS
Bateson defines twins as "the production of equivalent structures by division. The process may affect the whole body of an animal or plant or certain of its parts." In other words, bilateral doubling in some degree is a necessary feature of twinning. If this is so, as will be discussed later, the term twin, as ordinarily used, a term which refers merely to the simultaneous birth of two individuals, is not biologically correct.There are two kinds of twins: (1) Twins that are no more alike than any other two members