As in all of biology, comparative studies showing differences among species are often helpful for a better understanding of the basic mechanisms; with all its advantages, there is a danger of clinging exclusively to one model organism.

Any object, whether animate or inanimate, will have a size. Airplanes, boats, or musical string instruments vary in size just like animals and plants, and in all cases, their size and their material construction are totally different matters even though they affect one another.

That the role of size has been to some degree neglected in biology may lie in its simplicity. Size may be a property that affects all of life, but it seems pallid compared to the matter which makes up life. Yet size is an aspect of the living that plays a remarkable, overreaching role that affects life's matter in all its aspects.

There are good reasons why natural selection has become widely accepted as an explanation of evolutionary development. When applied to mammals and other large animals, it fits perfectly. But we cannot assume that all evolutionary steps arise from selection, particularly when looking at smaller animals.