426 THE SURVIVAL OP THE UNLIKE. [XXVI. 



vable varieties are found, with the single exception 

 of Texas, where another perplexing branch of the 

 group is native. 



About two hundred varieties of native plums are 

 now known and named. They are contending with 

 the European type for supremacy in the continental 

 basin, and, whilst still much inferior to the foreigners 

 in quality of fruit, they are destined to win in the 

 ceaseless struggle for existence. They possess great 

 superiority in what we call constitution. But they 

 have other merits which are quite as pronounced. I 

 refer to their comparative immunity from the attacks 

 of the black-knot and leaf-blight fungi, diseases 

 which are very serious upon the common plums. 

 These fungi are both native of this country. The 

 black -knot appears to have traveled chiefly from New 

 England westward, while the leaf -blight is invading 

 the northwestern plum lands from the west and south. 

 At first sight, it seems strange that our native plums 

 should be more immune from our native diseases 

 than the European plums are, but there is excellent 

 reason for it. It is plain that, in the course of the 

 evolution of the species of native plums, those forms 

 which were most susceptible to the attacks of these 

 fungi would be exterminated by them, whilst those 

 forms most immune would stand the best chance of 

 perpetuating their kind. In other words, there has 

 no doubt been a long and fierce battle between the 

 fungi and the plums and both have persisted, but 

 they have developed away from each other. But the 

 European plum, never having had this contest with the 

 two fungi named, is unprepared to meet their attacks. 

 We find this same comparative immunity from in- 



