upon the Earth.

Although we recognise in the instability of living systems the
underlying reason for their numerical abundance, secondary
evolutionary causes are at work. The most important of these is
the self-favouring nature of the phenomenon of reproduction. Thus
there is a tendency not only to favour reproductiveness, but
early reproductiveness, in the form of one prolific
reproductive.

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act, after which the individual dies.[1] Hence the wavelength of
the species diminishes, reproduction is more frequent, and
correspondingly greater numbers come and go in an interval of
time.

Another cause of the numerical abundance of life exists, as
already stated, in the conditions of nourishment. Energy is more
readily conveyed to the various parts of the smaller mass, and
hence the lesser organisms will more actively functionate; and
this, as being the urging dynamic attitude, as well as that most
generally favourable in the struggle, will multiply and favour
such forms of life. On the other hand, however, these forms will
have less resource within themselves, and less power of
endurance, so that they are only suitable to fairly uniform
conditions of supply; they cannot survive the long continued want
of winter, and so we have the seasonal abundance of summer. Only
the larger and more resistant organisms, whether animal or
vegetable, will, in general, populate the Earth from year to
year. From this we may conclude that, but for the seasonal
energy-tides, the development of life upon the globe had gone
along very different lines from those actually followed. It is,
indeed, possible that the evolution of the larger organisms would
not have occurred; there would have been no vacant place for
their development, and a being so endowed as Man could hardly

[1] Weismann, _The Duration of Life._

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have been evolved. We may, too, apply this reasoning elsewhere,
and regard as highly probable, that in worlds which are without
seasonal influences, the higher developments of life have not
appeared; except they have been evolved under other conditions,
when they might for a period persist. We have, indeed, only to
picture to ourselves what the consequence of a continuance of
summer would be on insect life to arrive at an idea of the
antagonistic influences obtaining in such worlds to the survival
of larger organisms.

It appears that to the dynamic attitude of life in the first
place, and secondarily to the environme