rm of one is fertilised by the male element of another, we may
believe that it is acted on in a somewhat similar manner as an individual
when exposed to slightly changed conditions. Now, every one must have
observed the remarkable influence on convalescents of a change of
residence, and no medical man doubts the truth of this fact. Small farmers
who hold but little land are convinced that their cattle derive great
benefit from a change of pasture. In the case of plants, the evidence is
strong that a great advantage is derived from exchanging seeds, tubers,
bulbs, and cuttings from one soil or place to another as different as
possible. {146}

The belief that plants are thus benefited, whether or not well founded,
has been firmly maintained from the time of Columella, who wrote
shortly after the Christian era, to the present day; and it now
prevails in England, France, and Germany.[323] A sagacious observer,
Bradley, writing in 1724,[324] says, "When we once become Masters of a
good Sort of Seed, we should at least put it into Two or Three Hands,
where the Soils and Situations are as different as possible; and every
Year the Parties should change with one another; by which Means, I find
the Goodness of the Seed will be maintained for several Years. For Want
of this Use many Farmers have failed in their Crops and been great
Losers." He then gives his own practical experience on this head. A
modern writer[325] asserts, "Nothing can be more clearly established in
agriculture than that the continual growth of any one variety in the
same district makes it liable to deterioration either in quality or
quantity." Another writer states that he sowed close together in the
same field two lots of wheat-seed, the product of the same original
stock, one of which had been grown on the same land, and the other at a
distance, and the difference in favour of the crop from the latter seed
was remarkable. A gentleman in Surrey who has long made it his business
to raise wheat to sell for seed, and who has constantly realised in the
market higher prices than others, assures me that he finds it
indispensable continually to change his seed; and that for this purpose
he keeps two farms differing much in soil and elevation.

With respect to the tubers of the potato, I find that at the present
day the practice of exchanging sets is almost everywhere f