1 66 Outline of Genetics 



also show the greatest change on crossing. There is at 

 least one difference between the two, however; in time 

 of maturity, environment and hybrid vigor have some- 

 what opposite effects. Generally speaking, favorable 

 growing conditions tend to delay flowering and maturing, 

 while conditions w^hich tend to stunt the plants tend, 

 like hybrid vigor, to hasten them (East and Jones 6). 



There seems little doubt that hybrid vigor is also manifested 

 in the animal kingdom. One might reasonably expect this from 

 the fact that the principles of inheritance are fundamentally the 

 same in plant and animal kingdoms, and hybrid vigor is a matter 

 of inheritance. As a matter of fact, there are many cases among 

 the records of professional animal breeders which might be cited 

 as evidence of hybrid vigor. It seems equally evident, however, 

 that this is not so general a phenomenon among animals as among 

 plants; and it should be noted that many zoologists refuse to 

 recognize in hybrid vigor anything like a general law, pointing 

 out cases among animals in which hybridizing apparently results 

 in loss of vigor. 



It is rather to be expected that such a general phenomenon 

 as hybrid vigor must have played a part in the evolution of the plant 

 kingdom. A few suggestions follow (from East and Jones 6) . 



1. Fixation of characters favoring cross-fertilization. — "Vari- 

 ations must have appeared that favored cross-fertilization. 

 Those plants producing a cross-fertilized progeny would have had 

 more vigor than their self-fertilized relatives. The crossing 

 mechanism could then have become homozygous and fixed, while 

 the advantage due to cross-fertilization continued." 



2. Fixation of sex act itself. — "Some means of favoring union 

 of dissimilar spores occurred as a chance variation. Through the 

 combination of somewhat different qualities this new dual product, 

 the zygote, was better enabled to develop and reproduce. Its 

 survival coefficient was high. The tendency for union of spores 

 persisted and became characteristic of the species." 



3. Preservation of undesirable characters in cross-fertilized 

 species. — "In self-fertilized species, new characters that weakened 



