ANIMAL AND PLANT BREEDING 321 



Very often, however, the desired character cannot be found in the 

 crop plant itself, or not in the best varieties of it, and the factor must 

 then be brought in from another species or variety. For instance, the 

 northern range of the orange in CaHfornia was greatly extended by 

 combining, as far as possible, the good quahties of the normal orange 

 with the cold resistance of the Chinese trifoliate orange, whose fruits 

 are themselves valueless. Another example of very valuable advances 

 obtained by wide hybridization is the work of the Russian breeder 

 Michurin^ with fruit trees. Michurin collected wild forms from the 

 enormous forests of plums, cherries, peaches, etc., which are found in 

 Persia and adjacent regions (their centre of origin, cf. p. 250), as well 

 as local races from the Far East. Crosses between these and European 

 forms have given some exceptionally cold-resisting types, which seem 

 likely to be of the greatest possible value in the north of the U.S.S.R. 

 One of his most spectacular successes is the production of a mountain- 

 ash (one of the hardiest of trees) with palatable berries. Michurin 

 developed a considerable number of practical aids to the carrying out of 

 such wide crosses, which are often difficult to bring off. He used a 

 technique of "vegetative rapprochement," attempting to overcome 

 interspecific sterility by bringing the plants vegetatively in contact, 

 either by grafting a whole branch of one on to the other, or by tying a 

 piece of the style of the pollen parent on to the style of the ovule parent. 

 The physiological basis of many of these methods, if they are in fact 

 successful, is not understood, but in the example given above it is 

 perhaps not impossible to see how the procedure might work. More 

 doubt attaches to Michurin's belief that a young hybrid is in a very 

 unstable condition, and can be altered, even in its genotype, by external 

 agents, e.g. by being attached, either as a stock or as a scion, to a graft 

 of some other species which acts as its "mentor." Very much more 

 evidence than is yet available would be necessary before this pre- 

 Mendelian type of concept can be accepted. Unfortunately Michurin 

 also believed that better results were obtained by the use of mixtures 

 of pollen, so that it is impossible to be certain of the nature of many 

 of his alleged hybrids. 



In recent years, Russian scientists in particular have been very active 

 in collecting from all over the world wild races or species which seem 

 to contain valuable mutations.^ It may be expected that the utilization of 

 this material will lead to great advances, though it is still too early for 

 much to have come of the collections as yet. An example of the richness 



^ Michurin, cf. Hudson 1937. - Rev. Hudson 1937. 



