282 



The Journal of Heredity 



port and with results identical with 

 those obtained by Castle, and Heape, 

 with guinea pigs and rabbits, respect- 

 ively. It is hardly necessary to sug- 

 gest that these results offer striking 

 evidence of the preponderant role 

 played by the germ plasm in heredity. 

 It appears that the germinal material 

 is peculiarly insulated from the body 

 as far as any influence on its capacity 

 as the carrier of the heritage goes. 



The causes of the appearance of the 

 two kinds of twins in man appear 

 now to be quite clearly understood, 

 due to the brilliant work of Galton, 

 Newman, and Patterson. The one 

 class of twins may consist of a boy 

 and a girl, or both may be of the same 

 sex. In either case, the one usually 

 differs from the other in respect to a 

 few, or even a great many, character- 

 istics, just as any brother may differ 

 from brother, sister from sister, or as 

 sisters from brothers. In this, the 

 more common kind of twinning, it 

 appears to have been quite definitely 

 ascertained that each individual is de- 

 rived from a separate egg, and as far 

 as heredity is concerned, the difference 

 between the two may be the same 

 as if they were born years apart. 

 Such twins as these I am sure you 

 have often seen, this one tall and thin 

 the other short and fat, the one with 

 black, the other with blue eyes ; the 

 one with curly, the other with straight 

 hair; the one with a certain disposi- 

 tion, the other entirely different, and 

 so on, as might be the case among 

 children born of the same ])arents at 

 the usual intervals. 



The other kind are designated as 

 identical twins. They are invariably 

 of the same sex, and are alike in prac- 

 tically all respects at birth. It ajjpears 

 well established that they are both 

 derived from the same fertilized Ggi:^ 

 cell. 



It was .Sir Prancis Galton, the 

 tatlicr of eugenics, who made the first 

 ini]jortant study of identical twins, 

 having collected accounts of eighty 

 pairs. The records of thirty-five pairs 



were given with remarkably complete 

 details. These records showed that 

 such twins presented extraordinary re- 

 semblances in childhood, and these 

 likenesses persisted to a remarkable 

 degree throughout life. You are un- 

 doubtedly familiar with one or more 

 pairs of such identical twins. They 

 invariably have the same bodily char- 

 acteristics, except in case of accident 

 before, at, or after birth, and their 

 mental and other traits appear also to 

 be identical. If they suffer no de- 

 forming accidents, and are dressed in 

 all respects alike, it is quite impossible 

 to distinguish between them. It has 

 been my fortune to become intimately 

 acquainted with two pairs of such 

 identical twins. In the one case two 

 small elementary school girls were ex- 

 actly alike, and dressed alike, and 

 although I taught them every school 

 day for several months, I never came 

 to the point where it was possible to 

 know them apart. Their resemblances 

 extended to mental traits also. Two 

 brothers were my college mates for 

 four years, living in the same house, 

 and part of the time in adjoining 

 rooms. Although they were my inti- 

 mate associates in many affairs, on 

 account of their similarity, extending 

 even to minute matters of habit, and 

 as they took pride in dressing alike in 

 all respects, I was never sure of their 

 individual identity. One of them had 

 spent some years with missionary 

 parents in China, while the other had 

 been with relatives in this country, 

 and a good portion of their lives had 

 been lived in different environments 

 including food and climate. 



Galton narrates numerous amusing 

 as well as serious anecdotes concern- 

 ing the eighty pairs of identical twins 

 whose histories he studied. Most of 

 these anecdotes related to the difficul- 

 ties of parents, relatives and tutors 

 and even of the twins themselves in 

 distinguishing between the members 

 of such pairs. In one case doubt re- 

 mained whedier the children were not 

 changed in their bath, and the pre- 



