The preceding chapters in this book have dealt with structures and functions and how they are related to body size. We have discussed bones and muscles, energy metabolism and oxygen supply, why time has a different meaning for a mouse and an elephant, and so on. We have considered animals that move about, running and jumping, swimming and flying, and how body size affects the energy cost of locomotion. This is important, for real animals do not sit around doing nothing; they spend much of their time moving about and being active. One important fact is evident: Although comparing animals at rest can provide a great deal of information, it is in the active animal that we are apt to find the limits and constraints on the various functions that make up the whole animal.

Wherever we look at the functions of living organisms, we find that size is important and that a change in size has consequences that require appropriate adjustments or changes. Various functions must be appropriately adjusted; they must be modified as dictated by a change in scale.

Some variables remain size-independent; for example, physical and chemical constants cannot be changed. That is, animals must find the best possible solutions within the existing limitations or rules as determined by the realities of the physical world. When animals meet constraints that set limits to further change in scale, discontinuities in design may solve the problem.