A recent article in Science discusses the application of contemporary engineering and mathematical concepts of dimensional analysis and physical similarity to biological objects.1 It is shown that so-called dimensionless numbers, which might be better called criteria of similarity, give a quantitative expression to certain geometric and physical ratios which remain constant in spite of changes in the gross size of the organism. The genuine constancy of the proposed biological similarity criteria is demonstrated by substitution of allometric (functions expressed as some power of mass of the animal) data. Such criteria may be said to show in what respects organisms and organs are models of each other.
Additional discussion is provided of how physical similarity criteria apply to artificial organs. The quite well-known Reynolds Number is useful in design and testing of small models of boats or planes, and it appears feasible to give dimension-less numbers which will serve the