Numerous slight
differences in characters and numerous external influences benefit the
minor types and bring them into competition with the better ones.
Sometimes they tend to supplant the latter wholly, but ordinarily sooner
or later a state of equilibrium is reached, in which henceforth the
different sorts may live together. Some are favored by warm and others
by cool summers, some are injured by hard winters while others thrive
then and are therefore relatively at an advantage. The mixed condition
is the rule, purity is the exception.
Different sorts of cereals are not always easily distinguishable by the
layman and therefore I will draw your attention to conditions in
meadows, where a corresponding phenomenon can be observed in a much
simpler way.
Only artificial pasture-grounds are seen to consist of a single species
of grass or clover. The natural condition in meadows is the occurrence
of clumps of grasses and some clovers, mixed up with perhaps twenty or
more species of other genera and families. The numerical proportion of
these constituents is of great interest, and has been studied at
Rothamstead in England and on a number of other farms. It is [103]
always changing. No two successive years show exactly the same
proportions. At one time one species prevails, at another time one or
two or more other species.
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