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Identical twins are more conspicuous members of society. They are the mirror images of each other and all the world's their stage. The product of a single egg which sometime in the course of its development has divided, they are much alike in biological construction, as they are in appearance. There is a greater similarity between the corresponding hand or foot of each twin, than between the hands or feet of either one.
The lower animals rarely indulge in this form of biological sport, with the exception of the Texas armadillo* which reproduces regularly in this way. It bears four young of the same sex, having a common set of embryonic membranes, strikingly similar in general configuration, all coming from a single egg. Thus they parallel as quadruplets the conditions of human identical twins. Exactly what causes the egg to divide is not definitely known. Dr. Horatio H. Newman, zoologist at the University of Chicago, has patiently pursued simpler species in the hope of finding a clue. After sacrificing several starfish he has shown twinning among echinoderms to be caused by retardation of development in the egg. Whether this is true of the human ovum remains to be proved. What is true is that identical twins are the closest approximation to different human beings with exactly the same heredity. They form the original rope for the tug of war between Heredity and Environment.
The third type of twins is the Siamese. These twins are the products of a single egg but, less fortunate than identical twins, they are not even separate individuals.
Psychological. Obviously twinship need have no mental effect on fraternal twins other than the general effect of being the same age and subject to similar environment.
Identical twins on the contrary are as alike mentally as physically whenever they are brought up together. Psychiatric literature is full of the case histories of identical twins stricken by the same psychoses at the same time. They have the same hallucinations, hear the same voices, suffer from the same delusions. No single instance has been found of one twin going insane while the other remained sane. Sometimes this has been shown to be the result of association, and separation in the ward has brought about changes in the character of the dementia. Would each have gone mad if the existence of the other had been unknown? There are doctors who believe they would; that having the same inheritance, developed from the same egg, the insanity is a proof of an inherited emotional instability that would have manifested itself at the same time whether the twins were together or apart. Others claim the mental disease to be the result of environment and association; insist at least that identical twins would not go simultaneously mad or have the same type of insanity if they had not been brought up together. This, in extreme form, is the field of the present investigation.