EECIPEOCAL CROSSES EVOLUTIONARY CHANGES. 59 



pure races are of no effect, entirely new characters which thus seem 

 to appear suddenly, but which in reality may have been thousands 

 of years in developing. Characters arising in this way would appear 

 only in heterozygotes if the two factors brought together happened 

 to form a pair. But if they did not form a pair the new character 

 would be capable of being fixed. 



Some phenomena have occurred, especially in crosses between 

 distinct species, which probably belong in the class here considered, 

 though not enough work has been done in this direction to make this 

 entirely certain. 



RECIPROCAL CROSSES. 



If in one cross we use the pollen of race A on the stigmas of race B, 

 while in another cross we use the pollen of race B on the stigmas 

 of race A, these two crosses are said to be reciprocal to each other. 

 Ordinarily such crosses give identical results. In his work with 

 hybrid wheats at the Washington Agricultural Experiment Station 

 the writer made reciprocal crosses in three instances, and in each 

 case the results of the reciprocal crosses were identical. There are 

 cases, however, where reciprocal crosses give different results. In 

 some species the plant produces more than one kind of pollen and 

 only one kind of ovule, as Correns found in Bryonia. He crossed 

 a dioecious species with a monoecious species. When he used pollen 

 of the dioecious species the hybrids were male and female in equal 

 numbers, but when he used the pollen of the monoecious species 

 the hybrids were all female. A number of other cases are known 

 which may be explained on a somewhat similar basis. In a few 

 cases differences have appeared in reciprocal crosses for which no 

 explanation has been found. The plant breeder should make a care- 

 ful record of any such cases coming under his observation, as they 

 may lead to important advances in our knowledge of the principles 

 of heredity. 



EVOLUTIONARY CHANGES AND THEIR RELATION TO PLANT 



BREEDING. 



While a great deal of study has been given to the general subject 

 of evolution, actual knowledge of how and why evolutionary changes 

 occur is very limited. The discussion this subject has received 

 has been largely theoretical, and usually in support of some theory 

 as to the manner in which such changes occur. 



We may perhaps distinguish two or more classes of evolutionary 

 changes. A complex organism is provided with many hereditary 

 characters — that is, characters which appear in successive generations. 

 These characters may change in the manner of expression. For 

 instance, a species having purple flowers may change with reference 



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