232 GENETICS AND EUGENICS 



nomic practice, the object of which is not the production of a 

 new breed, but the production for the market of an animal 

 maturing quickly or of superior size and vigor. The inbreed- 

 ing practiced in forming a pure breed has not of necessity 

 diminished vigor, but a cross does temporarily (that is in the 

 Fi generation) increase vigor above the normal. Now why 

 should inbreeding unattended by selection decrease vigor, 

 and crossbreeding increase it.^ We know that inbreeding 

 tends to the production of homozygous conditions, whereas 

 crossbreeding tends to produce heterozygous conditions. 

 Under self-pollination for one generation following a cross 

 (involving one unit-character only), half the offspring become 

 homozygous; after two generations, three-quarters of the 

 offspring are homozygous; after three generations seven- 

 eighths are homozygous, and so on. So if the closest inbreed- 

 ing is practiced there is a speedy return to homozygous, pure 

 racial conditions. We know further that in some cases at 

 least heterozygotes are more vigorous than homozygotes. 

 The heterozygous yellow mouse is a vigorous lively animal; 

 the homozygous yellow mouse is so feeble that it perishes as 

 soon as produced, never attaining maturity. Crossbreeding 

 has, then, the same advantage over close breeding that fer- 

 tilization has over parthenogenesis. It brings together differ- 

 entiated gametes, which, reacting on each other, produce 

 greater metabolic activity. East and Jones have suggested 

 that the superiority in vigor of crossbred over inbred indi- 

 viduals is roughly proportional to the number of genetic dif- 

 ferences between the races crossed. This idea is worthy of 

 an experimental test. 



Inbreeding, also, by its tendency to secure homozygous 

 combinations, brings to the surface latent or hidden reces- 

 sive characters. If these are in nature defects or weak- 

 nesses of the organism, such as albinism and feeble-minded- 

 ness in man, then inbreeding is distinctly bad. Existing 

 legislation against the marriage of near-of-kin is, therefore, 

 on the whole, biologically justified. On the other hand, 

 continual crossing only tends to hide inherent defects, not to 



