158 INBEEEDING AND OUTBREEDING 



material for this particular problem as plants, on account 

 of their bisexuality, as was explained earlier, and for this 

 reason but few quantitative data are available. There is 

 no question but that animals behave the same as plants 

 in heredity ; therefore, one might transfer the conclusions 

 reached in the one kingdom to the other without apology, 

 for the effects of inbreeding and cross-breeding are 

 wholly and solely the working out of the laws of heredity. 

 At the same time, it will not be amiss to present some of 

 the results obtained by zoologists, for they strengthen the 

 case immensely. 



In the two cultivated species of insects which form our 

 sole instances of domestication, bees and silkworms, there 

 seems to be evidence of increased vigor on crossing only 

 in silkworms (Toyama ^^^), In the fruit fly, however, upon 

 which the greatest amount of genetic work has been done, 

 Castle,2i Moenkhaus,^*^ Hyde^^ ^nd Muller^^* all found 

 size, fecundity and general constitutional vigor increased 

 remarkably, particularly when the strains crossed had 

 been inbred previously. In the rotifer, Hydatina, Whit- 

 ney 216 and A. F. Shull ^®® obtained similar results. Fur- 

 ther, Gerschler^® describes and figures first generation 

 crosses between different genera of fishes which show very 

 marked increases in size. 



In birds also there is such an increase in vigor that 

 poultry fanciers often cross two distinct strains and sell 

 the progeny because of their rapid growth and large size. 

 No attempt is made to breed from the hybrids ; they are 

 simply produced because of their vigor. When very great 

 differences in size exist, there is not, of course, an increase 

 in size sufficient to throw the individual of the first hybrid 

 generation above the larger parent, as is shown by the 



