374 HELEN DEAN KING 



Wentworth's ('13) experiments with Drosophila indicate that 

 the supposed weaknesses from inbreeding are due to "the mere 

 segregation of factors for lower vigor." Assuming that a similar 

 segregation of these factors occurred in the inbred rats during the 

 early generations, individuals containing the factors for 'lower 

 vigor' were evidently eliminated by the selective action of mal- 

 nutrition, and only those animals containing dominant genes for 

 'high vigor' were able to survive and to perpetuate their kind. 

 Neither inbreeding nor selection is creative in its action. Selec- 

 tion can act on fertility only by preserving those individuals that 

 contain genes for characters favorable to reproduction; inbreed- 

 ing conserves these characters, and, to a certain extent, intensifies 

 them. The action of both selection and inbreeding can be 

 nulhfied by unfavorable conditions of environment or of nutrition 

 which may produce a rapid deterioration in the fertihty of any 

 stock, regardless of the way in which the animals are bred. 



It was shown by the work of Darwin ('78), as well as by a 

 number of more recent experiments (Shull, '10; East and Hayes, 

 '12; Hayes and Jones, '17), that crosses between different varie- 

 ties of plants often produce hybrids that possess greater reproduc- 

 tive vigor than either parent stock. This result is due, according 

 to East and Hayes ('12), to "the stimulation of vigor through 

 heterozygosis." Inbreeding, these authors state, "tends to 

 isolate homozygous strains which lack the physiological vigor due 

 to heterozygosity. Decrease in vigor due to inbreeding lessens 

 with decrease in heterozygosity and vanishes with the isolation 

 of a completely homozygous strain." If the latter is a good 

 strain, because of its gametic constitutional and natural inherent 

 vigor, it is "ready to stand up forever under constant inbreeding." 



The results obtained in these inbreeding experiments with the 

 rat accord with the theory of East and Hayes to some extent. 

 The effects of inbreeding on the fertility and on the vigor of the 

 rats were obscured in the early generations by the action of mal- 

 nutrition, but it would appear that the animals lost very little of 

 their constitutional vigor during this time, since adequate nutri- 

 tion soon restored the normal productiveness of the strain and its 

 general vigor as well. Apparently at about the tenth generation, 



