412 Experimental Zoology 



putting the different facts together it seemed plausible, as I 

 have said, that identical twins arise in this way. 



There is a further fact that supports this hypothesis. Abnor- 

 mal embryos are sometimes born, composed of two individuals 

 united in various ways. 1 These are the so-called double mon- 

 sters, and in the cases in which the sexual prgans are separate 

 they are always of the same sex. 



In the case where there are three offspring at the same birth, 

 triplets, all three may be identical, or two may be identical and 

 the third only fraternal, or they may be all fraternal. The iden- 

 tical twins are of the same sex, and on the hypothesis they come 

 from one egg, while when two only are identical the third may 

 be of the same or of the other sex. There is at least one authen- 

 tic case in which all three individuals were of the same sex, 

 and when they grew up were closely similar. 



Jehring has confirmed a statement made on hearsay by Azara, 

 to the effect that in the armadillo (Tatusa hybrida) of Paraguay 

 the offspring of one birth are all of the same sex. In two preg- 

 nant females examined by Jehring all eight young were males. 

 Each embryo had its own aminon, but all were inclosed in a 

 common chorion. The latter fact is supposed to furnish con- 

 clusive evidence that the embryos must have arisen from the same 

 egg ; but while this is probable, it has not been shown that the 

 chorions of different individuals might not unite into a common 

 envelope. The same reasoning has been applied to the case 

 of identical human twins that are supposed to be always in- 

 closed in a common chorion. 



The idea that the two sorts of twins differ in the respects 

 noted above seems to have been noticed by Dareste in 1874, 

 and by Fisher in 1866; but Galton (1891) has especially drawn 

 attention to this difference. Recently Thorndike has disputed 

 the assumption that there is a sharp distinction between identical 

 twins and ordinary or fraternal twins. Thorndike finds as the 

 result of an examination of a large number of cases that there is 

 a perfect gradation between identical and fraternal twins. He 



1 Wilder (1904) has brought together the records of such cases. 



