PREFACE 



The present volume brings together for the first 

 time a considerable mass of data dealing with the 

 phenomenon of twins in man and other mammals. 

 Twins are so inherently interesting to so many people 

 that it is hoped by the writer that some light on how 

 twins "happen" will be welcomed by the general reader 

 as well as by the biologist. There are many thoroughly 

 interesting and nontechnical phases of twin-biology 

 that will appeal to anyone who is a twin or has personal 

 acquaintance with twins. There must be certain other 

 phases of the subject, however, that are largely of 

 value to the professional biologist. It has been the aim 

 of this book to satisfy both the general and the technical 

 reader without sacrificing unduly the demands of 

 simplicity on the one hand or of scientific adequacy on 

 the other. 



H. H. N. 



