sRcT. n PHYSIOLOGY 289 



will prevail in any cross, and the question can only be settled by experiment ; 

 usually the phylogenetically younger character appears to be dominant. 



When the parents differ in two characters instead of only one, mono- 

 hybrids instead of dihybrids result. It then appears that the several characters arc 

 independently transmitted and distributed in the descendants (autonomy of 

 characters). Thus new combinations of characters may come about, a fact of 

 great importance in plant-breeding. From the crossing of peas with yellow, 

 wrinkled seeds, and those with green, smooth seeds, among other possible 

 combinations of the characters the new ones yellow-smooth and green-wrinkled 

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

 characters). 



It is not possible to enter in this [)lace into the complicated phenomena of the 

 production and segregation of dihybrids and polyhybrids. 



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

 Thus in crossing a white-flowered with a yellow-flowered Mirahilis 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 cryptomerous races. 



The Mendelian 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 Silcnc 

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

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

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

 hybridisation, however, of nearly allied forms 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 ditt'erent genera Lychnis and Silcne, Rhododendron and 

 Azalea, Aexjilops and Triticicm, Secede and Triticiun (Rimpau), Zea and Euchlciena 

 (Zea canina results from a cross between Z. mais $ and Euchlaenct 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 



U 



