Danforth: Resemblance and Difference 



407 



ties that sometimes exist between bi- 

 ovular twins, considerable difficulty is 

 experienced in explaining the differ- 

 ences that occur in twins known, or 

 assumed, to be uniovular. Why are 

 these twins not actually identical? The 

 answer to this query must wait, but 

 there are a few points that may throw 

 some light on the question. In the first 

 place, it is a matter of common observa- 

 tion that the two sides of the same indi- 

 vidual are by no means identical. It 

 may be recalled that a few years ago 

 it was a fad with certain newspapers to 

 secure full-face views of prominent 

 people and, dividing them in the median 

 line, reproduce a left side to correspond 

 to the right and a right to correspond 

 to the left. The faked pictures were 

 then printed showing Mr. Blank as he 

 would look (a) if the left side of his 

 face were symmetrical with the right 

 and (b) if the right side were sym- 

 metrical with the left. The hair line, 

 the arch of the eyebrows, the shape of 

 the eyes, the ears, and the mouth, the 

 facial expression, were often very dif- 

 ferent in (a) and (b), so much so that 

 in some cases the two pictures would 

 hardly have been supposed to represent 

 uniovular twins. Anatomists are f?- 

 miliar with the rather frequent depart- 

 ures from symmetry in bilateral mus- 

 cles, nerves and vessels. Occasionally 

 individuals occur with one blue and one 

 brown eye, and indeed it is probable 

 that all symmetrical parts of the body 

 are subject to occasional unilateral 

 variations. The origin of these varia- 

 tions, however, is probably not en- 

 vironmental in the usual sense, since 

 here the environment factor is reduced 

 to its minimum. 



A\niatever may be the cause of varia- 

 tion between the two sides of the body 

 when they develop together as a single 

 individual, it is reasonable to expect 

 that they will be equally effective when 

 each half of the blastoderm develops as 

 a separate individual. It might there- 

 fore be predicted that uniovular twins 

 would differ from each other in the 



' See article by Margaret V. Cobb in Science N. S., vol. xli. No. 1057, pp. 501-502, 

 April 2, 1015. 



twins. In any statistical studies on 

 school children or inquiries addressed 

 to twins themselves this group tends to 

 become lost, a fact that was interest- 

 ingly brought out in the responses of 

 twins to the requests for photographs. 

 Of 160 pairs whose pictures were sub- 

 mitted 77 pairs were women or girls, 

 61 pairs were men or boys, and only 

 22 pairs were of both sexes, most of 

 the last being children whose pictures 

 were sent in by their parents. Statistics 

 based on thousands of cases show that 

 about one pair of twins in every three 

 pair born consists of a boy and a girl, 

 yet, despite the fact that the appeal 

 sent out to the newspapers asked for 

 pictures of twins irrespective of their 

 sex or resemblance, "pigeon" twins 

 were not sufficiently interested to co- 

 operate or to show curiosity to any 

 appreciable extent. Similarly, by an 

 easy deduction," it can be shown that 

 twins of the same sex should be uniovu- 

 lar in about two cases out of five, yet in 

 the opinion of the writer those twins 

 of like sex who submitted their pictures 

 show very striking resemblances in at 

 least three cases out of five. This seems 

 to indicate that even where both twins 

 are of the same sex the interest in each 

 other is roughly proportional to their 

 resemblance. 



The conclusion with reference to our 

 second question would seem to be that 

 biovular twins should be expected to 

 show resemblances in hereditary traits 

 ranging from practical identity to wide 

 divergence, and that in any ordinary 

 collection of photographs or data sup- 

 plied by twins themselves or their 

 friends they will seem to show an aver- 

 age degree of resemblance greater than 

 that for brotliers and sisters in general. 

 That is, of course, entirely apart from 

 any environmental influences that might 

 be supposed to still further heighten 

 their resemblances. 



WIIV ARE THERE DIFFERENCES IN 

 UNIOVULAR TWINS? 



While known facts furnish a basis 

 for understanding the marked similari- 



