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-cinctus, 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 twinsinman. 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 seven in 
which both twins suffered from some special ailment or had some 
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 
