24 THE TREND OF THE RACE 



While ordinary twins show varying degrees of resemblence, 

 identical twins belong apparently in a class by themselves. It is 

 a commonly accepted view, having much evidence in its favor 

 that true identical twins which are always of the same sex, are 

 developed within the same chorion and arise from the same ferti- 

 lized egg. They may therefore be regarded as having the same 

 heredity. Among armadillos, Dasypus novem-cinclus , it is known 

 that commonly four young are derived from a single ovum, which 

 develops beyond the gastrula stage before giving rise to four 

 embryos, and it is not improbable that a similar procedure is 

 occasionally followed in the development of twins in man. Double 

 monsters in man are of the same sex and are known in many cases 

 to have been enclosed in the same chorion, but it is unfortunate 

 that direct observational evidence that identical twins are in fact 

 monochorial is lacking although many facts support this conclu- 

 sion. The cases of remarkably close resemblance between twins 

 are so numerous that it is not reasonable to suppose that they are 

 the results of merely chance associations of similar ancestral 

 characteristics. Galton remarks that, "Among my thirty-five 

 detailed cases of close similarity, there are no less than se""'en in 

 which both twins suffered from some special ailment or had sonie 

 exceptional peculiarity. One twin writes that she and her sister 

 'have both the defect of not being able to come down stairs 

 quickly, which, however, was not born with them, but came on 

 at the age of twenty.' Three pairs of twins have peculiarities in 

 their fingers; in one case it consists in a slight congenital flexure 

 of one of the joints of the little finger; it was inherited from a 

 grandmother, but neither parents, nor brothers, nor sisters show 

 the least trace of it. In another case the twins have a peculiar 

 way of bending the fingers, and there was a faint tendency to the 

 same peculiarity in the mother, but in her alone of all the family. 

 In a third case, about which I made a few enquiries, which is given 

 by Mr. Darwin, but is not included in my returns, there was no 

 known family tendency to the peculiarity which was observed in 

 the twins of having a crooked little finger. In another pair of 

 twins, one was born ruptured and the other became so at six 





