18 



MORPHOLOGIC VARIATION 



of other populations, as fruit flies in a bottle or human populations 

 in a restricted area; and also to the growth curves of those co- 

 ordinated populations of cells which we call multicellular organisms. 

 This has been emphasized by Pearl (1925), who concludes that a 

 common law governs the growth of all such populations, which he 

 states as follows: 



"Growth occurs in cycles. Within one and the same cycle, and 

 in a spatially limited area or universe, growth in the first half of the 



Fig. 1. The Growth Curve of Bacteria. 

 From "Life Phases in a Bacterial Culture" by R. E. Buchanan. Reproduced 

 through the courtesy of the Journal of Infectious Diseases. (1918, 23, 109). 



cycle starts slowly, but the absolute increment per unit of time in- 

 creases steadily until the midpoint of the cycle is reached. After 

 that point the increment per unit of time becomes steadily smaller 

 until the end of the cycle. In a spatially limited universe the amount 

 of increase which occurs in any particular unit of time, at any point 

 of the single cycle of growth, is proportional to two things, viz.: 

 (a) The absolute size already attained at the beginning of the unit 

 interval under consideration, and (b) the amount still unused or 



