We must be careful, in seeking for relationships between different races, not to confuse pedomorphy and gerontomorphy with normal variations that take place when animals of the same or related species grow smaller or larger. A mouse has a larger brain, in proportion to its body size, than a rat does. A Great Dane's eyeballs are proportionately smaller, although absolutely larger, than those of a terrier.
Animals that are otherwise genetically similar vary in proportions according to size, the small ones being more compact, the larger ones more attenuated. The principle governing these differences is called allometry. Zoologists not only recognize this rule express it in formulas. For example, in the horse family face length equals .3 times skull length, to the 1.2 power.6 A big horse has a longer face, both absolutely and relatively, in proportion to his skull length, than a small horse does. By the same token, an average African Pygmy has relatively shorter legs and a relatively larger head than does an average African Negro.