316 BOTANY PART i 



of pairs of characters, the number of internally distinct combinations is 3 m , and the 

 number of constant forms 2 m . If there are four pairs of characters there thus 

 result 81 hybrid-combinations, and among these 16 constant forms. 



Many characters, however, tend to remain associated together (coupled 

 characters). 



In crossing many races quite new characters frequently appear in the hybrids. 

 Thus in crossing a white-flowered with a yellow-flowered Mirabilis there may result, 

 besides white-flowered and yellow-flowered descendants, others with rose-, red-, 

 red-white, yellow-white, rose-red, and rose-white coloured or striped flowers. 

 Since these newly appearing characters each in turn obey the Mendelian laws it 

 is assumed that they were latent in the parents, and that the latter belonged to 

 what are called crpytomerous races. 



The Mendeliau laws hold also in the animal kingdom and for the human race, 

 but only for the crossing of nearly related forms. Thus crosses between negroes 

 and negro-albinos follow Mendelian laws, while those between negroes and whites 

 do not. 



The more closely allied the parent plants, the more readily, as a rule, may 

 hybrids between them be produced. Many families seem to incline naturally to 

 hybridisation (Solanaceae, Caryophyllaceae, Iridaceae, etc.) ; others again develop 

 hybrids only occasionally or not at all (Papilionaceae, Convolvulaceae, Coniferae, 

 etc.). Even in the same family the related genera and species exhibit great 

 differences in the readiness with which they may be crossed. The Grape-vine, and 

 also the Willow are easily crossed with other species of their own genus, and the 

 same is also true of the different species of Dianthus, while the species of Silene 

 cross with each other only with difficulty. Species hybrids are easily produced 

 from species of Nieotiana, of Verbascum, and of Geum ; on the other hand, it is 

 very difficult to cross different species of Solanum, Linaria, or Potcntilla. The 

 hybridisation, however, of nearly allied iorms is often impossible the Apple with 

 the Pear, for instance although the Peach and Almond may be crossed, and also 

 the species of even the different genera Lychnis and Silene, Rhododendron and 

 Azalea, Aegilops and Triticum, Secalc and Triticum (RiMPAu), Zea and Euchlacna 

 (Zea canina results from a cross between Z, mais <J and Euchlaena mexicana ? ), 

 each according to their " sexual affinity." 



DERIVATIVE HYBRIDS arise when hybrids are crossed with one another, or with 

 one of the original parent forms. In this way it has been possible to unite six 

 species of Willow in one hybrid, and in the case of the Grape-vine even more species 

 have been combined. It is only in rare cases, however, that the form of the hybrid 

 remains constant in the succeeding generations. These exhibit more frequently a 

 tendency to revert to one of the original ancestral forms. 



In addition to their inherited qualities HYBRIDS EXHIBIT NEW 

 PECULIARITIES not derived from their parent forms. These are a 



MODIFIED FERTILITY, GREAT TENDENCY TO VARIATION, and often a 



MORE LUXURIANT GROWTH. The fertility is often so enfeebled that 

 the hybrids either do not flower (Rhododendron, EpiloUum), or are sterile 

 and do not reproduce themselves sexually. This enfeeblement of the 

 sexuality increases the more remote is the relationship of the ancestral 

 forms. Other hybrids such as those of Salix and Hieracium remain 

 fertile. 



The tendency to variability is often greatly enhanced in hybrids, 



