THE BIOLOGY OF TWINS 159 



of the armadillo rolling up into a ball (a pre- 

 rogative of the Little Armadillo) " are totally in- 

 applicable to this species, for the animal turns over 

 on its back and kicks viciously and effectively with 

 its powerful and heavily armed feet." But beyond 

 noting that the young ones are born well advanced 

 and able to walk about within the first few hours, 

 we must not say anything more about the natural 

 history of the Nine-banded Armadillo. For our 

 present purpose the important fact is that this quaint 

 creature normally and habitually produces quad- 

 ruplets a remarkable fact which several zoologists 

 have studied, and Mr. Newman most thoroughly 

 of all. 



Many mammals, such as rabbits, produce 

 numerous young ones at once, but each of these 

 develops separately from an egg-cell, and the 

 phenomenon of multiparous birth has nothing to do 

 with twinning. That term is appropriate when 

 a creature normally uniparous, such as cow or bat, 

 gives birth simultaneously to two offspring. As 

 every one knows, these may be quite dissimilar, or 

 they may be living images of one another, in which 

 case they are always of the same sex. In un- 

 toward cases twins may be physically continuous, 

 as in the " Siamese twins," and one member of the 

 pair may be to a varied degree degenerate or un- 

 developed, the result being the twin-monsters of 

 the show or of the embryological museum. Some 

 of them are the results of fission, others of fusion. 

 But there has been much vagueness and uncertainty 



