56
PERIODICITY IN ORGANIC LIFE.
This plant's appearance is a good example of the abundant
period. It will be remembered how alarmed those who had charge
of the canals were, and how they were at their wits' end, to find
out some means for reducing the mischief ere the canals were
rendered quite impassable. Our native water-plants were choked
and destroyed, and it seemed as if the new-comer was going to
appropriate all our waters, to the destruction of everything besides ;
but the period of prosperity is over; the plant is gradually becoming
less of a pest; our British plants are again able to assert and
maintain their proper place; our waters are becoming less clogged,
and let us hope that we are approaching the period of scarcity of
this species, which we could so well spare. How can the extreme
abundance of this species be accounted for, and how can its gradual
diminution be explained ? The waters are as before, their
ingredients are the same, and the surroundings have not altered, but
still Anacharis is apparently dying out. This may be a case of
periodicity, and if so, this water pest will some day again be growing
in the same rampant condition.
Who is there, dwelling in the country, of observant habits, that
has not noticed the abundance at uncertain intervals of the common
thistle. In those seasons it may be seen everywhere, not only in the
badly-cultivated and neglected fields, but also in pastures which
have been for years fed down by cattle only, and where the soil is
undisturbed also, and no manure put on. This condition may exist,
and does do so frequently, over large districts. After a time it may be
noticed that the thistles are less abundant, and then for years only
the normal quantity may be seen.
It must be understood I am not referring to such arable lands as
may be neglected and out of cultivation, for here weeds of all kinds
are sure to abound. Neither do I believe that this periodic
abundance is caused by the seeds that are blown from neglected
spots, because, some years since, when every part of Essex was
well and carefully cultivated, the extreme prevalence of this weed was
noticeable for a few seasons in succession. This periodic abundance
occurs in many plants, but the examples given may be sufficient to
illustrate my meaning.
All members of the Essex Field Club collecting any of the orders
of insects, must have noticed that for several successive years
certain species are rare, and that after an interval, longer or shorter,
they begin to find the individuals of these species more commonly,