viii PREFACE TO SECOND EDITION 



of crossing-over and the precocity theory of meiosis. In the 

 present edition I have therefore recast the whole account in terms 

 of evolution. I have shown mitosis as giving rise to meiosis and 

 sexual reproduction, diploidy to polyploidy in one direction and 

 to complex heterozygosis in another. I have represented structural 

 hybridity as the basis of sexual differentiation and I have deduced 

 the conditions of parthenogenesis from experimental observations 

 of the breakdown of sexual reproduction. 



Such a treatment leaves the other relationships of cell-behaviour 

 in the background. Of these the most important at the present 

 moment are the mechanical relationships. The special treatment 

 that I previously gave to evolution I now therefore devote to arrang- 

 ing our knowledge of chromosome behaviour and nuclear and cell- 

 division to reveal the laws of movement and enquire into their 

 causes. Ten years ago, in approaching these problems, we could 

 scarcely see the wood for the trees. Now order is beginning to 

 appear. Causal relationships are being established. The result, 

 I believe, is unmistakable. Cell structures, having a size inter- 

 mediate between those of molecular and macroscopic systems, 

 show properties of movement and development peculiar to their own 

 level of integration. These properties depend on the physiologic- 

 ally active contents of the nucleus, and since they are co-extensive 

 with heredity as we understand it, their study becomes a necessary 

 part of the investigation not only of the mechanics, but also of the 

 physiology of heredity. Thus in the cell, mechanics, physiology 

 and physical chemistry are being brought together to show the 

 unity of living processes. 



I have to thank Professor Haldane, Dr. Catcheside, Dr. Mather 

 and Miss Upcott for reading the proofs. I am indebted to Mr. La 

 Cour and Mr. Osterstock for many microphotographs, and to the 

 various authors acknowledged for the use of numerous text-figures. 



C. D. DARLINGTON. 



