THE SUGAR PRUNE 155 



Fruits of other species may ripen far earlier; 

 the cherry does so quite often. But the ancestors 

 of the plum have sometimes lived under con- 

 ditions that made it necessary for them to mature 

 their fruit much before midsummer. So their 

 range of habit in this regard, as recorded in the 

 stored hereditary tendencies, was limited. And 

 the possibihties of variation among these hybrid 

 seedlings are correspondingly limited, because, 

 as has hitherto been pointed out, heredity is but 

 the symbol of the sum of past environments, and 

 the hereditary limitations of any conmion race of 

 plants to-day are somewhat restricted by the 

 aggregate limitations of all their ancestors. 



Reveesion to the Avebage 



Such an analysis, in which the varying con- 

 ditions that environ the different strains of a 

 hybrid's ancestry are kept constantly in mind, 

 serves to give us a clue to the observed tendency 

 of families or strains of animals or plants to 

 revert in successive generations toward a given 

 mean or average. 



It has long been observed that, as a general 

 rule, the offspring of human parents that are 

 exceptionally tall tend to be shorter than their 

 parents; whereas, contrariwise, the offspring of 

 dwarfs tend to be taller than their parents. 



