234 PLANT HYBRIDIZATION BEFORE MENDEL 



all of the older school of breeders, that it was probable that 

 crossing in and of itself, when one or both the parents have been 

 long under cultivation, increases the "variability" of the offspring, 

 independently of the fact of the commingling of the parental 

 characters themselves, (ic, 2:243-4.) 



The fundamental cause underlying variation Darwin con- 

 sidered to be the food supply. 



"of all the causes which induce variability," he says, "excess of food 

 whether or not changed in nature, is probably the most powerful." 

 (ic, 2:236.) 



However, in face of the fact of the bud variation of the peach 

 to form the nectarine, Darwin concluded that there must have 

 been some cause, internal or external, to stimulate a bud to 

 change its character. He says : 



"l cannot imagine a class of facts better adapted to force on our minds 

 the conviction that what we call the external conditions of life are in 

 many cases quite insignificant in relation to the particular variation, in 

 comparison with the organization or constitution of the being which 

 varies." (ic, 2 : 269.) 



So, from the case of the red Magnum Bonu7n plum, which ap- 

 peared on a forty-year old tree of the yellow Magnum Bonum 

 variety, Darwin also concludes : 



"when we reflect on these facts, we become deeply impressed with 

 the conviction that in such cases the nature of the variation depends but 

 little on the conditions to which the plant has been exposed, and not 

 in any special manner on its individual character, but much more on 

 the inherited nature or constitution of the whole group of allied beings 

 to which the plant in question belongs. We are thus driven to conclude 

 that in most cases the conditions of life play a subordinate part in 

 causing any particular modification ; like that which a spark plays, when 

 a mass of combustibles bursts into flame, the nature of the flame de- 

 pending on the combustible matter, and not on the spark." (ic, 2:272.) 



In general, regarding the character of hybrids, Darwin held 

 that while in the majority of cases, the hybrid offspring are inter- 

 mediate between their parents, yet that certain characters are in- 

 capable of fusion. 



"when two breeds are crossed, their characters usually become inti- 

 mately fused together; but some characters refuse to blend, and are 

 transmitted in an unmodified state, either from both parents or from 

 one." (ic, 2 : 67.) 



As cases in point, Darwin cites the crossing of gray and white 

 mice, the offspring of which are pure white or gray, but not inter- 



