88 INBEElEDING AND OUTBREEDING 



inbreeding, and some idea of the composition of the indi- 

 vidual may often be had from a careful consideration of 

 the composition of the generation to which it belongs. 



In an endeavor to demonstrate the effect of various 

 systems of inbreeding upon Mendelian constitution, and to 

 appraise the effect of this constitution upon develop- 

 mental vigor, let us approach the problem from the op- 

 posite direction. 



It has been established that the effect produced by 

 crossing depends more or less closely upon the genetic 

 diversity of the types which produce the hybrid. The 

 usual result of crossing organisms which differ in many 

 characters is a first generation which is no more variable 

 than the parental types. The second generation, how- 

 ever, may be expected to show a greater variability be- 

 cause of Mendelian segregation. The amount of such 

 variability is a measure of the diversity of the parents 

 which produce the cross. It is in crosses which show 

 greater variability in the second generation that hybrid 

 vigor is expected in the generation immediately following 

 the cross. When such hereditary combinations are com- 

 posed of unlike elements, hybrid vigor is commonly 

 shown; when all the combinations are composed of like 

 elements hybrid vigor is absent. Hence, in crossed species 

 of wild or domesticated animals and plants part of their 

 vigor may be the result of dissimilar hereditary factors 

 acting together. If conditions are brought about by which 

 this dissimilarity in aUelomorphic combinations is re- 

 duced or lost completely a partial diminution of develop- 

 mental energy will occur. Since there is a constant 

 tendency for inbreeding of whatever kind to bring about 

 similarity in germinal construction, inbreeding will, 



