viii PREFACE 



groups of animals. A considerable number of interesting 

 situations, such as hemibj^Dertrophy in man, bilaterality 

 in echinoderm larvae, symmetry reversal, double limbs 

 and tails, supernumerary organs, and certain types of 

 tumors, turn out to be phases of twinning. This all 

 seems to mean that twinning is a much more general phe- 

 nomenon than we have previously supposed. Whether 

 it occurs in worms or in man, it expresses itself in 

 the same series of t3^es; and there is a deep-seated 

 coherency and consistency about its varied expressions. 

 It is because twinning is beheved to be a consequence of 

 one of the most fundamental of biological processes that 

 the present volume has been written, in the hope that 

 some of the riddles of Hfe may at least partially be 

 answered. 



I wish to express my thanks to my friends. Professor 

 F. R. LilHe and Dr. A. W. Bellamy, for their help in 

 reading the manuscript and for valuable suggestions, 

 and to Mr. Kenji Toda, who has so skilfully drawn 

 or redrawn the illustrations. 



H. H. Newman 



April lo, 1922 



