THE MUTABLE UNIT OF HEREDITY 35 



value. The present genotype is likely to be nearly the best for existing 

 circumstances. Consider a Swiss watch: to hit it with a hammer is more 

 likely to impair its function than to improve it. 



Nonetheless, we may suppose that evolution continues with the selec- 

 tion of improvements in the hereditary material, by the reorganization 

 of the whole genotype and through changes that occur in the environ- 

 ment. That advantageous mutations do occur is a well-established fact. 

 In the fruit fly, for example, some mutants are inferior in the environ- 

 ment in which they arose but superior in other environments. The way 

 in which new species arise is another story which will not be treated in 

 this book. It is part of the science of evolution, which, through the 

 use of genetic techniques, has become one of the most active and inter- 

 esting branches of biology. 



In the next chapter we shall take up the way in which differences are 

 transmitted from parent to offspring; this chapter is concerned with the 

 origin of observed variation. Novel characters of a heritable kind arise 

 as a consequence of sudden changes in the genetic material. Such muta- 

 tions are rare. It is thought that one took place in Queen Victoria of 

 England. Although there is no record of hemophilia among her an- 

 cestors, she transmitted a factor for that disease to her offspring. Not 

 much has yet been done to study the mutation process in man. Experi- 

 mentation is somewhat easier in organisms such as the fruit fly, Droso- 

 phila, which has a generation time of about two weeks and can be 

 inbred by brother-sister mating for many generations to yield individuals 

 with almost identical genetic constitutions. In such pure lines, a new 

 type having, say, white eyes instead of red may suddenly appear with 

 a frequency of about one in a hundred thousand. 



Mutation may be studied much more conveniently among organisms 

 which give rise by asexual reproduction to billions of offspring in one 

 day, as do bacteria and viruses. Such microorganisms are also con- 

 venient for chemical studies and, as we shall see, have genetic systems 

 basically similar to those in all other organisms. As a matter of fact, the 

 possession of genetic material which can mutate is a prerequisite for life. 



THE PARTICULATE NATURE OF THE MUTABLE UNIT 



Mutation is a heritable change in the genetic material; a mutant is 

 the changed organism that results. The standard of reference is the 

 wild-type condition, the state of organisms as they are found in nature. 

 Obviously this definition is arbitrary because even in nature organisms 

 and their hereditary materials vary. But once some condition has been 



