The development of language in twins: II. The development of twins: their resemblances and differences

has for a long time been very curious about their origin, their resemblances and their differences. The dispute as to the relative importance and effects of heredity and environment in moulding human nature dates back to the ancient Greek philosophers and scientific workers always have felt that twins presented a special opportunity for making such studies. Newman's (16) investigations on the star fish and the Texas armadilla as well as his more recent studies of twins, point unquestionably to the fact that there are 2 types of twins. The identical or monozygotic twins arise from the splitting of a single zygote while the fraternal or dizygotic twins arise from the simultaneous development of two zygotes. The psychological studies of twins beginning with that of Galton (4) in 1883 also give evidence of the two types of twins, in that they show a greater degree of resemblance in both physical and mental traits in identical twins than

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