512 EVOLUTION, GENETICS, AND EUGENICS 



Wistar Institute are entirely contrary to those of the workers just 

 mentioned. This piece of work was carried out in a most precise 

 manner, with large numbers of individuals. The original stock con- 

 sisted of four rather undersized but otherwise normal albino Norway 

 rats. Brothers and sisters were mated throughout the experiment. 

 For six generations no selective mating was practiced, with the re- 

 sult that many of the defects previously noted were in evidence; but 

 after the sixth generation some twenty females from about a thousand 

 were selected for their superior qualities. From this stock the result 

 of inbreeding for twenty-five generations was very good. Dr. King 

 seems to have produced an essentially homozygous race of white rats 

 that are superior in many ways to the race from which they have been 

 derived. It seems probable that selection has rid the race of all or 

 nearly all of the residual recessive characters, so that the present com- 

 bination is highly normal and standard. Sewall Wright, under the 

 auspices of the United States Department of Agriculture, has carried 

 out a very extensive program of inbreeding with guinea pigs. His 

 results are more in harmony with those of earlier workers than with 

 those of Dr. King. In general, the result of brother- and sister-mating 

 was a steady loss of vigor both bodily and reproductive. Both prena- 

 tal and postnatal mortality was increased. Some families, however, 

 remained quite strong after long inbreeding; while other families de- 

 clined so rapidly that it was impossible to perpetuate them after a few 

 generations. Some strange combinations of traits appeared in differ- 

 ent stocks. One stock was characterized by very low vitality, but 

 remained normal in body size and in number of young produced. 

 Another stock showed undiminished vitality but greatly lowered re- 

 productivity and reduced size. The chief difference in method used 

 in these two modern experiments seems to be that only the best were 

 bred in Dr. King's experiments, while in Dr. Wright's experiments no 

 such precautions were taken, probably because he preferred to approxi- 

 mate natural conditions. 



A large amount of experimentation in inbreeding has been carried 

 out with domestic animals and plants of all sorts, and the results have 

 shown as much diversity as those already reported; consequently, 

 opinions are still at variance as to the question whether inbreeding is 

 injurious per se. D. F. Jones, who seems to have given this matter 

 very careful consideration, takes the position that "the only injury that 

 comes from inbreeding comes from the inheritance received. ^^ If the indi- 

 viduals inbred possess many vmdesirable recessive characters, nothing 



