LUTHER BURBANK 



ing in this regard the best-protected of their wild 

 ancestors. 



Obviously, then, the condition of spinelessness 

 in the cactus has quite different relations in the 

 scheme of heredity from the conditions that gov- 

 ern spinelessness in the blackberry. In the latter 

 case, as we have seen, the spineless condition ap- 

 pears to be recessive, and the thornless individual 

 is as free from tendency to produce thorns as if 

 its entire coterie of ancestors had been perfectly 

 smooth-stemmed. The individual spineless cac- 

 tus, on the other hand, retains the factors for 

 spines in its germ plasm, to make their influence 

 tangibly felt in a large proportion of the offspring. 



Nevertheless, it does not appear that the con- 

 dition of spininess acts as a simple Mendelian 

 dominant. On the contrary, it appears that the 

 hereditary conditions that govern the spiny con- 

 dition in the cactus are very complex. The best 

 interpretation would seem to be that there are 

 multitudes of actors for spicules and spines, vari- 

 ously blended in the germ plasm of any given 

 individual. The spiny condition, on the whole, 

 tends to be dominant to the spineless condition, 

 because the spines are a relatively late develop- 

 ment in the history of the evolution of the cactus 

 tribe. 



But doubtless the period in question was an 



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