372 THE SURVIVAL OP THE UNLIKE. [XXIII. 



area of its cultivation enlarges, the enemies find it, 

 and it may turn out to be as liable to injury as any 

 of the older varieties, and, like them, it may fail for 

 this reason. In other words, absence of injury to a 

 new variety may not indicate immunity from disease, 

 but simply escape from it. 



I am inclined to believe, also, that a variety may 

 change in its relationship to disease, and possibly to 

 insect attack. May it not be true that many of the 

 so-called blight -proof pears really are measurably 

 immune, and that after a time they become suscep- 

 tible to attack? It is true, no doubt, that some 

 varieties of pears are freer from attack than others ; 

 that is, the species, the pear, varies within itself in' 

 this particular. Now, the variety differs from the 

 species in degree only, not in kind ; it is variable 

 within itself, and there is no philosophical reason 

 why it may not acquire new habits. More than this, 

 the behavior of many varieties of various plants in 

 reference to disease appears to indicate some such 

 change in character. How many are the old seedling 

 pear trees which, standing near affected ones, rarely 

 or never blight, but whose offspring blight as badly 

 as other kinds ! The same variety of plant often 

 behaves differently in different parts of the country 

 in reference to the same disease. The difference in 

 amenability to disease in different varieties of the 

 same species is admirably shown in the tomato. The 

 little - improved sorts, like the Cherry and Plum 

 tomatoes, are not attacked by fruit -rot, but the large 

 modern varieties are seriously affected. In other 

 words, if we had no large tomatoes we should 

 probably fear no such disease as tomato fruit -rot. 



