15 



the beginning of a new race or variety would readily be conceivable. 

 It is this factor of sexual sterility which must often be invoked to 

 explain the continued existence, side by side, of closely related species. 



If, on the other hand, the peculiar offspring shows no tendencies to 

 sexual sterility toward its parents or near relatives, but is transported 

 to some region which is separated from its home by a sufficient barrier, 

 there will be no opportunity for interbreeding with the parent stock, 

 and the new variety may be perpetuated. 



Both of these forces, among others, are unquestionably at work in 

 nature, and upon them certainly in part depends the origination of 

 species and varieties. The breeder recognizes the importance of pre- 

 venting the intercrossing of varieties which he has produced by care« 

 fully isolating individuals with especially desirable characters, and real- 

 izes how quickly a new variety will often be swamped if allowed to 

 interbreed. Inbreeding is a recognition of this principle. 



It will be evident from what has been said that in the former case 

 two nearly related species developed side by side and prevented from 

 interbreeding by sexual sterility* will be of comparatively little value 

 for crossing purposes, while two species owing their origin to geo- 

 graphical segregation may be quite fertile toward each other, and will 

 thus often prove of the greatest value in the production of new plant 

 varieties or hybrids. 



While the scope of this bulletin is too general to admit of a more 

 detailed discussion of this most important phase of plant introduction 

 in its relation to plant breeding, a few examples of its api^lication may 

 be cited to advantage. 



Mr. Luther Burbank,! of Santa Eosa, CaL, in the origination of his 

 remarkable hybrid blackberries and raspberries employed no less than 

 37 species of the genus Rubus, collected from all parts of the world. 

 His '' Japanese Golden Mayberry," the earliest rasx)beny known, 

 ripening in California before strawberries, is the result of crosses 

 between well-known American sorts, such as the Cuthbert and Euhns 

 palmatiis. This latter plant is described as an unproductive species with 

 quite insignificant berries, and not at all palatable, while the raspberry- 

 blackberry hybrid '^ Primus," remarkable for its early ripening, pro- 

 ductiveness, and large size of fruit, was obtained by crossing the 

 Western dewberry {BuMis ursinus) with a Siberian raspberry {Eubus 

 cratoegifolius). 



The new breeds of cereals which have lately attracted considerable 

 attention in England, the creations of J. & G. Gartou, of Newton-le- 

 Willows, have been produced by the employment of 7 different species, 

 represented by over 350 varieties of wheat, 3 species or 100 varieties 



* There are of course many other hindrances to interbreeding, snch as difference 

 in time of blooming^ and differences of floral structure and coloration affecting the 

 preferences of insect visitors. 



t Burbank, Luther. New Creations in Fruits and Flowers. Jane, 1893, p. 23, 

 ^anta Rosa, Cal. 



