PREFACE 



There are two ways of attempting to describe a part of 

 nature in scientific terms. One is to deal with the area which has 

 been exactly mapped by experiment, with the ensuing generaliza- 

 tions and predictions, and to leave the rest empty. The other is to 

 go further and use our knowledge of the mapped area to fill in 

 the empty spaces according to the more likely assumptions. The 

 first method is evasive, the second hazardous. We prefer the 

 second and have adopted it. In the present account of Genetics we 

 have tried to use what is known in order to find out what is 

 unknown. 



As a consequence there stretches a complete range, in a gentle 

 gradient or cline, from the old theories of Chapter i, which are 

 called Laws of Nature, to the new theories of Chapter i6, which are 

 called Dangerous Speculations, The reader will don his doubting 

 glasses at the point he feels proper. He will do well, however, to 

 recollect that this is the first attempt to represent the whole scope 

 of genetics, the whole of what has always been needed. In the past 

 open hypotheses have been replaced by concealed assumptions and 

 such assumptions are far more dangerous than the statements we 

 have laid before the reader in black and white. 



Events have lent a special urgency to the survey of the whole 

 territory of genetics. They have at the same time made it a venture 

 of special opportunity, one which offers unprecedented rewards. 

 Genetics has now reached the point when its central position in the 

 world of science is becoming generally understood. This isthmus 

 is being found to join continents that have hitherto been unknown 

 to one another. Across it now botanists and zoologists may venture 

 to find common ground with bacteriologists and virologists. On 

 its pathways the student of evolution may teach, and learn from, 

 the investigator of cancer and the practical stock-breeder. Within 

 its confines the physical chemist may verify some of his predictions 

 and confound others. Breathing its air, the physiologist and the 

 embryologist may come to agree that plants are organisms not too 

 simple to be used in explaining the more elaborate mysteries of 

 animal life. 



