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 



phase^/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^n the one hand or of scientific adequacy on 



the other. 



H. H. N. 



Vll 



