DO VARIETIES WEAR OUT? 179 



at their conclusion. For, as to the facts, it is not enough to 

 point out the diseases or the trouble in the soil and in the at- 

 mosphere, to which certain old fruits are succumbing, nor to 

 prove that a parasitic fungus (Peronospora infestans) is what 

 is the matter with potatoes. For how else would constitu- 

 tional debility, if such there be, more naturally manifest itself 

 than in such increased liability or diminished resistance to 

 such attacks ? And if you say that, anyhow, such varieties 

 do not die of old age, — meaning that each individual at- 

 tacked does not die of old age, but of manifest disease, — it 

 may be asked in return, What individual man ever dies of old 

 age in any other sense than of a similar inability to resist in- 

 vasions which in earlier years would have produced no notice- 

 able effect ? Aged people die of a slight cold or a slight acci- 

 dent, but the inevitable weakness that attends old age is what 

 makes these slight attacks fatal. 



Finally, there is a philosophical argument which tells 

 strongly for some limitations of the duration of non-sexually- 

 propagated forms, one that Knight probably never thought 

 of, but which we should not have expected recent writers to 

 overlook. When Mr. Darwin announced that the principle of 

 cross-fertilization between the individuals of a species is the 

 plan of nature, and is practically so universal that it fairly 

 sustains his inference that no hermaphrodite species continu- 

 ally self-fertilized would continue to exist, he made it clear to 

 all who apprehend and receive the principle, that a series 

 of plants propagated by buds only must have a weaker hold 

 of life than a series reproduced by seed. For the former is 

 the closest kind of breeding. Upon this ground such varie- 

 ties may be expected ultimately to die out ; but the mills of 

 the gods grind so exceeding slow that we cannot say that 

 any particular grist has been actually ground out under hu- 

 man observation. 



If it be asked how the asserted principle is proved or made 

 probable, we can here merely say that the proof is wholly in- 

 ferential. But the inference is drawn from such a vast array 

 of facts that it is wellnigh irresistible. It is the legitimate 

 explanation of those arrangements in nature to secure cross- 



