346 THE POPULAR SCIENCE MONTHLY. 



influence of their identical nurtures ; having the same home, the same 

 teachers, the same associates, and in every other respect the same 

 surroundings. 



My materials were obtained by sending circulars of inquiry to 

 persons who were either twins themselves or the near relations of 

 twins. The printed questions were in thirteen groups; the last of 

 them asked for the addresses of other twins known to the recipient 

 who might be likely to respond if I wrote to them. This happily led 

 to a continually- widening circle of correspondence, which I pursued 

 imtil enouo;h material was accumulated for a general reconnaissance 

 of the subject. 



There is a large literature relating to twins in their purely surgical 

 and physiological aspect. The reader interested in this should con- 

 sult "Die Lehre von den Zwillingen," von L. Kleinwiichter, Prague, 

 1871 ; it is full of references, but it is also disfigured by a number of 

 numerical mis^^rints, especially in page 26. I have not found any 

 book that treats of twins from my present point of view^ 



The reader will easily understand that the word "twins" is a 

 vague expression, which covers two very dissimilar events; the one 

 corresponding to the progeny of animals that have usually more than 

 one young one at a birth, and the other corresponding to those double- 

 yolked eggs that are due to two germinal spots in a single ovum. 

 The consequence of this is, that I find a curious discontinuity in my 

 results. One would have expected that twins would commonly be 

 found to possess a certain average likeness to one another; that a few 

 would greatly exceed that degree of likeness, and a few would greatly 

 fall short of it ; but this is not at all the case. Twins may be divided 

 into three groups, so distinct that there are not many intermediate 

 instances; namely, strongly alike, moderately alike, and extremely 

 dissimilar. When the twins are a boy and a girl, they are never 

 closely alike ; in fact, their origin never corresponds to that of the 

 above-mentioned double-yolked eggs. 



I have received about eighty returns of cases of close similarity, 

 thirty-five of which entered into many instructive details. In a few 

 of these not a single point of difierence could be specified. In the re- 

 mainder, the color of the hair and eyes was almost always identical ; 

 the height, weight, and strength were generally very nearly so, but I 

 have a few cases of a notable difference in these, notwithstanding the 

 resemblance was otherwise very near. The manner and address of 

 the thirty-five pairs of twins are usually described as being very simi- 

 lar, though there often exists a difference of expression familiar to 

 near relatives but unperceived by strangers. The intonation of the 

 voice when speaking is commonly the same, but it frequently happens 

 that the twins sing in different keys. Most singularly, that one point 

 in which similarity is rare is the handwriting. I cannot account for 

 this, considering how strongly handwriting runs in families, but I am 



