350 ENVIRONMENT AMONG MEN 



germinal constitution, whereas other twins would have germinal 

 constitutions as different as on the average are those of the off- 

 spring of the same parents. That identical twins do thus at 

 times arise is practically certain, but it has recently been shown 

 that the supposed sharp distinction between identical and other 

 twins does not exist.^ This may be explained by various assump- 

 tions. It may be supposed that some ova are binucleate or 

 that some ova are penetrated by two spermatozoa ; again the 

 earlier or later separation of the blastomeres may have a bearing 

 upon the position. Under any of these circumstances twins 

 intermediate between identical twins and ordinary twins would 

 arise — twins, that is to say, not having approximately similar 

 germinal constitutions, but germinal constitutions more nearly 

 alike than those of ordinary twins. However this may be, it is 

 sufficient for our purpose to note that so-called identical twins 

 are always more alike in their germinal constitutions than are 

 ordinary twins and may quite frequently have practically similar 

 germinal constitutions. It, therefore, follows that differences 

 between identical twins must be very largely of the nature of 

 modifications. It is thus of great interest to ask in what these 

 differences consist. 



Galton collected data regarding thirty-five cases of identical 

 twins. He summarizes the information about them up to the 

 time at which they left the family circle as follows. ' In a few 

 of these not a single point of difference could be specified. In 

 the remainder, the colour of the hair and eyes were almost always 

 identical ; the height, weight, and strength were nearly always 

 so. Nevertheless I have a few cases of a notable difference in 

 height, weight, and strength, although the resemblance was 

 otherwise very near. The manner and personal address of the 

 thirty-five pairs of twins are usually described as very similar, 

 but are accompanied by slight differences of expression, familiar 

 to near relatives though 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.' ^ He goes on to 

 say that * both twins are apt to sicken at the same time in no 

 less than nine out of the thirty-five cases. Either the illnesses, 



1 See Fisher, ' Genesis of Twins', Genetics, vol. iv. * Galton, Inquiries into 



Human Faculty, p. 219. Curiously enough there was no similarity to be detected 

 in handwriting — an interesting commentary on the value to be attributed to 

 deductions drawTi from handwriting as to character. 



