8 THE PHYSIOLOGY OF TWINNING 



fission is an integrating thread running through the 

 whole fabric of our twinning theories. 



THE ORDER OE PRESENTATION OF DATA 

 ON TWINS 



In general, the order of chapters is based on the 

 relative simphcity or the fundamental character of the 

 twinning processes. The simpler and more fully under- 

 stood cases of twinning, such as those in the starfishes 

 and earthworms, come first because it is believed that 

 there the phenomenon of twinning is less obscured by 

 developmental complexities. The most obscure and 

 least understood cases of twinning, as that in man, are 

 reserved for later consideration in order that we may 

 have as much information as possible about the modes, 

 causes, and consequences of twinning before tackling 

 the most difficult of our problems. In dealing with 

 twinning in the vertebrate classes the phylogenetic order 

 is followed because this order seems to lead from the 

 simpler to the more complex phases of the subject. 

 The causes of twinning in the armadillos of the genus 

 Dasypus seem to me to be more definitely known than 

 those of any other vertebrate, but the restatement of 

 this situation is postponed until after the consideration 

 of the twinning conditions in the birds and in the reptiles. 



The study of the literature on twins and double 

 monsters in man brought to light two rather unexpected 

 and extraordinary situations that seem to me to throw 

 much light on matters formerly quite obscure to biolo- 

 gists. The first of these situations has to do with the 

 various developmental hazards of human twins, especially 

 one-egg twins, due largely to interinfluences of the two 



