198 PLANT PHYSIOLOGY AND ECOLOGY 



a certain character found in one parent does not occur in the 

 other. In the second case the cross is termed bisexual, certain 

 characters of the parents combining in pairs. In unisexual crosses 

 the hybrid may show all the characteristic features of both parents, 

 some individuals resembling one more than the other. The hybrid 

 individuals may resemble one parent more than the other, even 

 to the extent of being scarcely distinguishable from it. In bi- 

 sexual crosses between a species with a certain character present 

 or dominant and a variety of it, in which this character is latent 

 or recessive, hybridation takes place in accordance with Mendel's 

 law. All of the first generation of hybrid individuals show the 

 dominant character of the species. If these plants are self-fertilized, 

 approximately three fourths of the second generation show the 

 dominant character, while one fourth exhibits the recessive charac- 

 ter of the variety. If the flowers are again self-pollinated, the 

 recessive individuals are found to come true to type. The domi- 

 nant group splits, some of the plants remaining dominant while 

 the others show the hybrid character of the preceding generation, 

 i.e., their progeny will contain recessive, dominant, and hybrid 

 forms. 



The production of new forms by hybridation occurs only when 

 the resulting hybrids are in some degree a mixture of the charac- 

 ters of the parents. It seems not to be a frequent source of evolu- 

 tion in nature, though a few distinctive forms are known to have 

 originated in this manner. 



Experiment 58. The occurrence of new forms in nature. Make a 

 careful scrutiny of the species of the flora for the purpose of discovering 

 ecads, variants, mutants, and hybrids. Note the differences between 

 the parents and the new forms discovered. Estimate the chances the 

 new forms have of surviving, using number, vigor, kind of modification, 

 etc., as a basis for this. 



211. Natural selection. The success of some individuals and 

 the handicapping or destruction of others in the process of natural 

 selection is due either to the action of physical factors or to com- 

 petition. Since the latter really operates through the reaction 

 upon the habitat, natural selection rests finally upon the ability 

 of a variation or change to cause a form to thrive in a particular 

 habitat. In both mutation and hybridation, the new form appears 

 suddenly, and selection can have nothing to do with its origin. 



