THE ORIGIN OF NEW FORMS 



197 



accumulated within the plant. The difference between a variant 

 and a mutant is apparently one of degree and not of kind. In 

 the one case, however, natural selection plays a necessary part 

 in the process, while with mutants it merely determines whether 

 they shall persist. The features of a mutant may be unfavorable 

 as well as favorable. In addition, a mutant crosses readily with 

 the parent form. In consequence, it runs many chances of disap- 

 pearing or of being merged with the original form. When the 



Fig. 70. Diagrams of flower "sports" or mutations in the firoweod, 

 Chamcenerium angustifolium. 



infrequence of mutants is taken into account in connection with 

 these facts, it seems probable that origin by mutation plays but 

 a minor part in evolution. 



2IO. Origin by hybridation. When two individuals more or less 

 unlike are cross-pollinated, the result is a hybrid. When the parent 

 individuals belong to the same form, crossing merely produces 

 ordinary fertilization. If the plants are quite different, i.e., if 

 they belong to distinct genera, the foreign pollen is as a rule unable 

 to produce fertilization. Hybrids ordinarily arise between related 

 species, between a species and its varieties, or rarely between 

 varieties. In the first case crosses are said to be unisexual, i.e., 



