102 SCIENCE PRIMERS. [xxvi. 



difficulty is-met/foy the consideration that no great 

 departure from the parent form (which has itself sur- 

 vived) can, * as a rule, be suited to the conditions of 

 the area; and that variation can have but narrow 

 limits during any one or a few generations, just as 

 the changes in natural conditions are slight during 

 short periods. 



155. Hybrids are the result of the ovules of one 

 species having been fertilized by the pollen of another. 

 ' They are called mules, and are rare in nature but 

 easily produced by art. Many grow rapidly and 

 flower copiously, but do not fertilize their ovules, 

 owing to the imperfection of these or of their pollen ; 

 hence they rarely ripen seed. On the other hand, 

 they often seed abundantly when fertilized by the 

 pollen of one of their parents. 



By hybridizing many more valuable results in a 

 horticultural point of view are produced than even 

 by cross-fertilizing (Par. 151); a scentless species, if 

 fertilized by the pollen of a scented one, may produce 

 a scented hybrid ; and by hybridizing, the qualities of 

 size, form, and colour of flower, fruit, and leaf, hardi- 

 ness, early or late flowering, &c., may be combined 

 in plants whose parents each possessed one only of 

 these qualities. 



XXVI. PHYSIOLOGICAL EXPERIMENTS. 



156. Absorption and evaporation of water. 



Take up three plants of the buttercup, carefully by 

 the roots; leave one (No. i) on the table; place 

 another (No. 2) with its roots in water; hang the 



