In the seventeenth century, it was held by some that inside a human sperm there was a minute human being - a homunculus - that was planted inside the womb. Development consisted of the miniature homunculus enlarging and passing through birth and on to maturity-just like inflating a balloon.
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.
Evolution, cell biology, biochemistry, and developmental biology have made extraordinary progress in the last hundred years - much of it since I was weaned on schoolboy biology in the 1930s. Most striking of all is the sudden eruption of molecular biology starting in the 1950s.
Changes in size are not a consequence of changes in shape, but the reverse: changes in size often require changes in shape. To put it another way, size is a supreme regulator of all matters biological. No living entity can evolve or develop without taking size into consideration. Much more than that, size is a prime mover in evolution.
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.
The reason for natural selection's great success is that it provides a satisfying explanation of how evolution might have occurred: individual organisms vary, and if those variations are inherited, the successful ones will survive and propagate and pass down their desirable traits to succeeding generations.
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.
When, as an undergraduate, I began experiments on these slime molds in 1940, only one other person, Kenneth Raper, was working on them at that time. In fact, he discovered the model species Dictyostelium discoideum, which is the species used in the majority of the experimental work today.