PHYSIOLOGY 289 



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 Nicotiana, of Verbascum, and of Geum ; on the other 

 hand, it is very difficult to cross different species of Solarium, Linaria, or Poten- 

 tilla. 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 different genera Lychnis and Silene, Rhododendron 

 and Azalea, Aegilops and Triticum, 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 are sterile and do not reproduce themselves sexually. 

 "This enfeeblement of the sexuality increases the more remote is the 

 relationship of the ancestral forms. The tendency to variability is 

 often greatly enhanced in hybrids, especially in those arising from 

 the hybridisation of different varieties of the same species. Hybrids, 

 particularly those from nearly related parents, produce more vigorous 

 vegetative organs, they bloom earlier, longer, and more profusely than 

 the uncrossed plants, while at the same time the flowers are larger, 

 more brilliant, and exhibit a tendency to become double. The luxu- 

 riance of growth and the increased tendency to produce varieties 

 displayed by the hybrids have made the whole subject of hybridisation 

 one of great practical as well as theoretical importance. 



It is doubtful if hybrid forms can be produced (graft-hybrids) by a 

 vegetative union of portions of two different plants (grafting, budding). 

 It will seem very improbable, as in all properly regulated experiments 

 the vegetatively united forms have preserved their independent in- 

 dividuality (p. 227). 



Alternation of Generations 



In the lower Cryptogams, as well as in the Phanerogams, vegeta- 

 tive and sexual reproduction may exist, either side by side or 

 following one another often in apparently irregular succession. After 

 many generations have been produced in a vegetative way, in the 

 case of the Algae or Fungi, sexual organs suddenly appear ; but by 

 both modes of reproduction descendants of similar appearance are pro- 

 duced. Although in this case sexually and vegetatively produced 

 generations succeed each other, it would not, strictly speaking, be 



