PREFACE 



This book deals primarily with the causes and conse- 

 quences of twinning. In 191 7 was published a volume 

 belonging to this series and written by the present 

 author, entitled The Biology of Twins. That volume was 

 limited to twinning in mammals and was largely mor- 

 phological in character. The word ''twins" was viewed 

 broadly so as to include one-egg (monozygotic) as well as 

 two-egg (dizygotic) twins. We now realize that true twin- 

 ning is essentially a matter of the division of a single egg 

 at various stages of development and that dizygotic twin- 

 ning is only a case of plural offspring born simultaneously. 

 The present volume is confined to one- egg twinning and, 

 instead of being limited to mammals, includes all known 

 types of complete or partial one-egg twinning in the 

 animal kingdom. In The Biology of Twins very Httle 

 attention was paid to theories as to the causes of twins 

 or to the interinfluences of twins upon each other. One 

 definite theory, however, was advanced by the writer 

 to account for the curiously unique process of twin- 

 formation in the armadillos. This theory was the first 

 real causal theory of twinning based on facts. Inas- 

 much as this theory is now supported by a great body of 

 evidence from many sources, the writer, at the risk of 

 appearing too soHcitous about ma:tters of priority, feels 

 impelled to lay definite claim to its authorship. This 

 theory is the center and substance of the present book. 

 When used as a working hypothesis it bids fair to explain 

 a long array of peculiar twinning processes in diverse 



