DO VARIETIES WEAR OUT? Lie 
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- 
