90 EVOLUTION AND ANIMAL LIFE 



between the parent stocks renders hybridization impossible. 

 But among plants, when hybrids are actually formed, fertility 

 rather than sterility may be taken as the rule. This is the case 

 with Mr. Luther Burbank's Primus berry, a cross between the 

 Siberian raspberry (Rubus cratcegifolius) and the Calif ornian dew- 

 berry or blackberry (Rubus ursinus). In this form the fruit 

 excels in size and abundance either parent, and the hybrid 

 breeds true from the seed, and ripens before either parent begins 

 to bloom. It was fixed in the first generation, being in this re- 

 gard a rare exception to the general rule of the aberration of 

 hybrids. In this and in other respects the Primus, known to 

 be an intentional cross of two species, behaves as though it 

 were a distinct species. In like fashion, the Logan berry, the 

 product of an accidental cross at Santa Cruz, in California, of 

 the European raspberry with the native dewberry, behaves also 

 like a distinct species, and is also much superior in productive- 

 ness to either parent. 



The fine art of the horticulturist is seen in the selection and 

 fixing of the variations produced by crossing and hybridization. 

 While most of the forms thus obtained are worthless, a few 

 will show decided advances. Often as m.uch progress may be 

 made in a single successful cross or hybridization as in a dozen 

 or even a hundred generations of pure selection. 



By selection alone, however, important results may be 

 obtained, with time and patience. Given a variation in a de- 

 sired direction there is perhaps no actual limit bounding the 

 possibilities of selection unless arising through external or me- 

 chanical conditions. Thus selection for speed of horses is limited 

 by the strength of the material of which a horse's leg is com- 

 posed. The increase in the number of petals may be limited 

 by the space on which petals can stand, and the number of 

 leaflets in a leaf by the length of the rhachis. Still there are 

 known cases in which a positive limit has been reached in at- 

 tempting to modify organisms by selection alone. 



Accidental crossing within a species may form a useful basis 

 for selection. Thus from the seeds in a single potato ball of the 

 Early Rose variety, crossed by insects with an unknown parent, 

 Mr. Luther Burbank reared potatoes of many different sorts: 

 red potatoes, white potatoes, elongate potatoes, potatoes rela- 

 tively smooth and potatoes all eyes and "eyebrows." Among 

 all these, one form, long, white, smooth, and mealy, seemed far 



