INTRODUCTION 



field, this ecological approach is both indispensable and 

 fascinating. 



More recently, however, I have gradually become im- 

 pressed with the way a related but somewhat simpler concept 

 has seemed to pervade much of what I have been thinking 

 about. This is the ecological and evolutionary implications 

 of the clone — as contrasted with the sexually reproducing 

 population. This interest began, I think, in 1942, when Bull 

 and I first described what happens when an influenza A virus 

 is transferred to the cavities of the chick embryo. There is 

 a strange and striking shift of characters in the influenza 

 virus during that process — and from that time onward I have 

 been deeply interested in the genetics of animal viruses. 

 Though most of my work in recent years has been concerned 

 with recombination between strains, something that is almost 

 a primitive fusion and redistribution of genetic material, 

 I still regard the clonal aspects as the more basic and it is 

 with these that I shall mostly be concerned. 



Along with an interest in virology, I have always found 

 a fascination in the nature of the immune process. 



It is irrelevant how my views on the nature of antibody 

 production have changed over the years. What is relevant is 

 the growing opinion that in a real sense the development of 

 specific antibody-producing capacity is something charac- 

 teristic, not so much of a cell as of a clone of cells. 



Such a point of view sets one thinking more generally of 

 the mesenchymal cells of the body, that enormous population 

 of fibroblasts and lymphocytes, histiocytes and granulocytes, 

 cells of variable, often short life, expendable cells produced 

 in large amount or discarded according to the requirements 

 of the body. We can be certain that their form and function 

 may change according to the impact of the internal environ- 

 ment, even if the histologists still seem to be at variance as to 

 which type of cell can give rise to or be converted into which. 

 Surely, here we find also a situation where clones of cells 

 flourish and wane in very much the same fashion as bacterial 



