INTRODUCTION 



clones do in a culture. And there is the challenge to see 

 whether this concept may not be fruitful for an understanding 

 of the multitude of physiological and pathological changes 

 within the mesenchymal cell populations. 



And this in its turn leads naturally to the classic problem 

 of human pathology, the nature of cancer. There are many 

 ways of looking at cancer and at the present time 'somatic 

 mutation' is an unpopular phrase. But I think that whether 

 we are interested in cancer viruses, carcinogens, or ionizing 

 radiation, we come in the last analysis to clones of cells 

 differentiated by their reproductive behaviour from the 

 normal cells of the body. A year ago I attempted to present 

 a simple description of the cancer process along these lines 

 and, though I doubt if it convinced anyone, some readers 

 found it stimulating. In this series of lectures I hope to lead 

 from a discussion of immunological topics and some aspects 

 of the pathology of the mesenchymal cell system, to a re- 

 capitulation of this clonal approach to malignant disease. 



All the topics I am going to consider can be expressed in 

 a simple generalized form. We have a population of cells or 

 micro-organisms which reproduce wholly or predominantly 

 by asexual methods. In a varying environment, changes in 

 the character of the population take place. In every instance 

 we are concerned to understand (a) whence the change in 

 character of the individual components arises and (b) the 

 nature of the process by which the constitution of the popula- 

 tion is modified to conform to the requirements of the 

 changing environment. 



This is in essence the oldest of biological problems — the 

 classic struggle between the Lamarckians and the Darwinians 

 on the nature of adaptation in evolution. It is a theme that 

 has been played upon in an infinity of variations over the last 

 100 years, and I can only hope that in the present treatment 

 there are scattered elements of novelty, either in regard to 

 example or approach, which will make up for the inevitable 

 inclusion of much that is trite and familiar. 



