The weather during the spring and summer
benefits some and hurts others, the winter may be too cold for some, but
again harmless for others, the rainfall may partly drown some species,
while others remain uninjured. Some weeds may be seen flowering
profusely during some years, while in other summers they are scarcely to
be found in the same meadow. The whole population is in a fluctuating
state, some thriving and others deteriorating. It is a continuous
response to the ever changing conditions of the weather. Rarely a
species is wholly annihilated, though it may apparently be so for years;
but either from seeds or from rootstocks, or even from neighboring
lands, it may sooner or later regain its foothold in the general
struggle for life.
This phenomenon is a very curious and interesting one. The struggle for
life, which plays so considerable a part in the modern theories of
evolution, may be seen directly at work. It does not alter the species
themselves, as is commonly supposed, but it is always changing their
numerical proportion. Any lasting change in the external conditions will
of course alter the average oscillation and the influence [104] of such
alterations will manifest itself in most cases simply in new numerical
proportions. Only extremes have extreme effects, and the chance for the
weaker sorts to be completely overthrown is therefore very small.
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