314 THE CONTINUITY OF THE RACE 



time to see the evidence that the same sorts of variations are continuously 

 coming into existence in many (probably in all) races of organisms, from 

 which we must infer that all the allelic qualities available to the Men- 

 delian breeder have arisen in the same way in the past. 



The best evidence that a given inherited quality is actually new and 

 was derived by some spontaneous change in the genotype of one of its 

 recent ancestors is provided by stocks of experimental organisms that 

 have been bred for many generations under laboratory observation. 

 These stocks have a long history of continued inbreeding 1 that has elimi- 

 nated any chance that an unknown ancestor might have introduced some 

 hidden recessive into the germ plasm or that the appearance of a new 

 character might have been caused by a new combination of genes already 

 present. When the laboratory stock of the fruit fly Drosophila melanogaster 

 was started, the "wild" parents were red-eyed. This normal red-eyed 

 character persisted through many inbred generations before a white-eyed 

 individual suddenly appeared in this purebred red-eyed stock. The new 

 variant, a male, was bred to a red-eyed female, and the white-eyed condi- 

 tion reappeared in the inbred descendants of their offspring. White-eyed 

 males and females were then bred together to establish a homozygous 

 white-eyed strain of fruit flies. White-eyed proved to be a sex-linked 

 recessive to red-eyed. 



During the half century or so that Drosophila has been bred in the 

 laboratory, hundreds of pedigreed generations and millions of individuals 

 have been carefully scrutinized by the geneticist. Among this huge num- 

 ber of individuals more than 500 new variants have appeared, each new 

 character proving to be an allele of some formerly homozygous character 

 of the inbred stock. Similar though less numerous instances of the sudden 

 appearance of new inherited characters have occurred in nearly all the 

 stocks of organisms that have been intensively studied by the experi- 

 mental breeder — numerous strains of mice, rabbits, poultry, insects, and 

 plants. The field naturalist and the practical animal and plant breeder 

 have likewise encountered hundreds of new variations that seem un- 

 doubtedly to be instances of the same phenomenon, though experimental 

 proof is lacking. 



The term mutation is applied and often restricted to the appearance 

 of a new variation due to the origin of a new gene. 2 There is abundant 



1 Inbreeding is the mating of close relatives and results in progeny that have fewer 

 than the theoretical maximum of recent individual ancestors. The mating of cousins, 

 for example, reduces the total number of separate great-grandparents from eight to 

 six, and the mating of brother and sister reduces the number of grandparents from 

 four to two. The intensity of inbreeding varies from a maximum with self-fertiliza- 

 tion to a minimum with cousin mating, with numerous intermediate degrees, but all 

 degrees of inbreeding tend to disclose any recessive genes that may exist in the stock. 



3 Originally, the term mutation was applied to the appearance of any new, inheritable 



