162 INBREEDING AND OUTBREEDING 



possible a larger number of offspring. The degree to 

 which heterosis is expressed is correlated, within limits, 

 with the differences in the uniting gametes. When homo- 

 zygous forms are crossed, it is at its maximum in the 

 first hybrid generation, and diminishes in subsequent gen- 

 erations of inbreeding as segregation occurs and homo- 

 zygosity is again attained. It is a widespread phenom- 

 enon and accompanies heterogeneity of germinal consti- 

 tution whether the organisms crossed are from the same 

 or diverse stocks, whether they have been produced under 

 similar or under different environmental conditions; 

 although it is not apparent until the zygote is formed, 

 from that time on it is expressed in many ways through- 

 out the lifetime of the individual and is undiminished 

 by asexual propagation. 



These are the effects of cross-breeding upon develop- 

 ment in which we have been particularly interested, those 

 in which the organizations of the combining gametes are 

 sufficiently compatible to permit continued propagation. 

 But it must not be forgotten that we have dealt with only 

 one part of the problem. As the differences between the 

 forms increase limits are reached beyond which the organ- 

 isms neither reproduce nor flourish. One can arrange a 

 series in plants in which (1) the parents are so diverse 

 the cross cannot be made; (2) the seed obtained fails to 

 germinate under any set of conditions; (3) the hybrids 

 are so weak they are unable to reach maturity; (4) the 

 hybrids are extremely vigorous, but sterile except pos- 

 sibly in back-crosses ; (5) the hybrids are fully fertile and 

 more vigorous than either parent; or (6) the parents may 

 be so closely related no effects whatever are to be noted. 



