314 ILLINOIS STATE ACADEMY OF SCIENCE 



tween the eruption of teeth in the two babies may be 

 as long as four months. 



Francis Galton made a study of twins from the bio- 

 logic and genetic aspects, and hoped to be able to differ- 

 entiate between the effects of tendencies received at birth 

 and of those that were imposed by the special circum- 

 stances of their afterlives. He sent questionnaires to 

 persons who were either twins or near relatives of them. 

 He received eighty replies, thirty-five of which entered 

 into instructive details. In a few of these not a single 

 point of difference could be specified. The color of the 

 hair and the eyes was almost always identical. In many 

 instances the twins were of the same height, weight, and 

 strength. In others there was a notable difference in 

 these factors, though the resemblance was in other re- 

 spects close. 



Gesell reports twins who showed superior intellect. 

 They both sat up at six months, walked and talked at 

 eleven months, learned French at three years, and were 

 in the seventh grade at the age of nine. They resembled 

 one another, both physically and mentally. In fact, 

 there was no noteworthy distinction between the two. 

 Their physique, countenance, demeanor, conversation 

 were completely similar. 



Homologous twins usually show marked similarity 

 physically, as well as mentally. They resemble each 

 other in weight, body structure, voice, and gait. They 

 may learn to speak and walk at the same time. Not 

 rarely they show the same anomalies and faults of de- 

 velopment. They may become sick at the same time and 

 die almost at the same time. 



Ganther and Rominger studied the finger prints of 

 five pairs of one ovum twins and forty-two pairs of two 

 ovum twins.. They found that in the five pairs of one 

 ovum twins there was marked similarity in the finger 

 prints, and in the system of lines of the hands. In the 

 two ovum twins there is a certain similarity of structure, 

 but never the striking correspondence of the system of 

 lines. They conclude from their studies that a striking 

 similarity of the structure of the lines of the hands indi- 

 cates that the twins are uni-ovular. 



