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nor humidity; nor excess or deficiency of electricity ; nor, in fact, any 
of the known physical agents constituting the power of the climate, will 
account for the propagation of the cholera, any more than they will of 
the potato rot. Why not assume, then, the degeneracy of the human 
race as a predisposing cause of cholera, as well as to assume that fact in 
regard to the potato to account for its rot? The one assumption, it ap¬ 
pears to us, would be just as logical and philosophical as the other. The 
impaired constitutional vigor of the potato is assumed as a starting point, 
and disease is predicated upon that condition of the plant. The argument 
is, that the impaired constitutional vigor, which limits the duration of the 
individual plant, is transmitted through the cuttings and tubers, and re¬ 
appears in the extended plant with the same fatal virulence as in the pa¬ 
rent stem. Nature has assigned, say the advocates of the theory, differ¬ 
ent periods of existence to different species of plants, as well as to dif¬ 
ferent races of animals. It is a law irrevocably and inexorably ordained 
that none shall live forever. The period of existence varies according to 
the constitutional power and tendencies of different plants. The oak, the 
lordly denizen of the forest, bows to the same inexorable decree as the 
ephemeral dust of the parasite propagated upon its branches. It is a law 
of nature that each shall spring into existence, fructify and die, ‘as a 
species.’ 
Admitting all this to be true—how is the theory of propagation by ex¬ 
tension affected by it ? If the potato, considered as a species, is doomed 
to degeneracy, decay and ultimate extinction, how is the propagation of 
it to be extended beyond the period assigned by nature ? The foundation 
of the proposition upon which the ‘wearing-out’ theory is based, is as 
impregnably laid against the renewal of vitality by the seed, as against 
the extension of it by the tubers. If the duration of the potato as a spe¬ 
cies, is as inevitably fixed as that of an individual plant, there is no such 
thing as abrogating or evading the decree assigning the determinate pe¬ 
riod. Seeds are as inadequate and powerless to that end as tubers. But 
suppose we apply the theory again to varieties instead of species, and as¬ 
sume that the debilitating effects of age are felt, after a certain period, by 
each succeeding plant continued from the tubers of a given variety. Does 
the extermination of that particular variety, or of its individual life, de¬ 
pend upon the method of cultivation? If each succeeding plant has a 
less hardy and vigorous constitution than that which preceded it, and if 
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