336 THE SURVIVAL OF THE UNLIKE. [xX. 



among themselves in hardiness and \iuor. as named 

 varieties differ from seedlings. The seedlings which 

 have survived in waysides and old plantations have 

 been able to do so, no doubt, because they were con- 

 stitutionally fitted to survive. No one knows how 

 many seedlings have perished because of weakness, 

 and it is certainly not fair to compare those varieties 

 which we chance to cultivate with those wild or half- 

 spontaneous individuals which have chanced to be 

 able to endure all vicissitudes. We ai-e fond of say- 

 ing that the farther the species departs from its 

 original or wild type, the weaker it becomes, but we 

 have no proof for such statement. 



Indeed, the facts cited in support of cultural de- 

 generacy prove conclusively another law, which I 

 may call varietal difference, or the proposition that 

 varieties differ widely among themselves in consti- 

 tution. It is well known, for instance, that some 

 varieties are much hardier than others, and then the 

 question invariably arises if the weaker varieties tend 

 to disappear, or to "run out." This question is too 

 broad for discussion here, but it may be said that in 

 apples, under common methods of propagation, wc 

 have little or no reason to believe that varieties are 

 self -limited. (Consult Essay XXIV.) 



I am strongly of the opinion, therefore, that the 

 failure in apple trees is due to external rather than to 

 internal causes. The reasons which my correspond- 

 ents have given for this failure may be named and 

 classified as follows : 



I. Extra -cultural causes. 



1. Change in climate. 



2. Greater abundance of insects. 



3. Greater abundance of fungi. 



