If we lay out waves as above to a common scale of time for
different species, the difference of longevity is shown in the
greater or less number of vibrations executed in a given time,
_i.e._ in greater or less "frequency." We cannot indeed draw the
curvature correctly, for this would necessitate a knowledge which
we have not of the activity of the organism at different periods
of its life-history, and so neither can we plot the direction of
the organic line of propagation with respect to the
[1] Projecting upon the axes of time and energy any one complete
vibration, as in Fig. 4, the total energy consumed by the
organism during life is the length E on the axis of energy, and
its period of life is the length T on the time-axis. The mean
activity is the quotient E/T.
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axes of reference as this involves a knowledge of the mean
activity.[1]
The group of curves which follow, relating to typical animals
possessing very different activities (Fig. 5), are therefore
entirely diagrammatic, except in respect to the approximate
{Fig. 5}
longevity of the organisms. (1) might represent an animal of the
length of life and of the activity of Man; (2), on the same scale
of longevity,
[1] In the relative food-supply at various periods of life the
curvature is approximately determinable.
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one of the smaller mammals; and (3), the life-history of a cold
blooded animal living to a great age; _e.g._ certain of the
reptilia.
It is probable, that to conditions of structural development,
under the influence of natural selection, the question of longer
or shorter life is in a great degree referable. Thus, development
along lines of large growth will tend to a slow rate of
reproduction from the simple fact that unlimited energy to supply
abundant reproduction is not procurable, whatever we may assume
as to the strength or cunning exerted by the individual in its
efforts to obtain its supplies. On the other hand, development
along lines of small growth, in that reproduction is less costly,
will probably lead to increased rate of reproduction. It is, in
fact, matter of general observation that in the case of larger
animals the rate of reproduction is generally slower than in the
case of smaller animals. But the rate of reproduction might be
expected to have an important influence in determining the
particular periodicity of the organism. Were we to depict in the
last diagram, on the same time-scale as Man, the vibrations of
the smaller and shorter-lived living things, we would see but a
straight line, save for secular variations in activity,
representing the progress of the species in time: the tiny
thrills of its units lost in comparison with the yet brief period
of Man.