organic. By
organic I mean the conditions which depend upon the number of
competitors or enemies by which a bird is surrounded. The competitors
may include other species which require a similar environment; and the
enemies, species which prey upon it, or animals which take its young or
its eggs. They vary in different seasons, in different districts, and
in nature and extent--the success of one species leads to the failure of
another, and the multiplication of the Jay or of the Magpie robs us of
many a songster.

By inorganic I refer to the changes in the climate and in the surface of
the earth. The nourishment of the young depends upon a regular supply of
food, and the supply of food depends upon the climate which alters in
different periods; in one decade the temperature falls below, whilst in
another it rises above, the normal, and, as the insect life fluctuates,
so there is fluctuation in the bird population. The changes in the
surface of the earth are manifold. Little by little the alder (_Alnus
glutinosa_) overspreads the marsh. Young shoots spring up here and
there, in a few years grow into bushes, and in a few more years are
trees; and the dense masses of rush which seemed to choke their growth,
yielding their position of importance, slowly disappear. And where
formerly the _Orchis latifolia_, _Orchis mascula_, and _Juncus communis_
grew in mingled confusion, nothing but water, moss, and the spreading
roots of alder cover the ground. As the rush disappears, many birds that
for generations have inhabited that marsh must seek accommodation
elsewhere. Ancient breeding haunts thus disappear, new ones come into
being, and even those which appear to be permanent are almost
imperceptibly changing.

Now the bird inherits a nervous system, which works under internal
excitation and external stimulation. Given the appropriate organic
condition and adequate stimulation, and the impulse to seek isolation
comes into functional activity. What the organic condition is and how it
arises we do not exactly know; all we know is that organic changes do
take place in the breeding season, that these changes profoundly modify
character, and that they correspond with the seasonal growth of the
sexual organs. And with regard to the question of stimulation, we have
again to confess to much ignorance, although certain facts are presented
to observation which seem to indicate the direction in which the
stimulus lies. For example, it is we