[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.
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
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.
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.
The interdependence of the rate of reproduction and