Fashions in Growth

THE BIOLOGY OF POPULATION GROWTH, Raymond Pearl, Knopf ($3.50). Dr. Pearl, Director of the Institute of Biological Research at Johns Hopkins, long famous in scientific circles for his studies of how living things grow, has here produced a book which is intelligible to the layman yet includes enough scientific and mathematical data to be significant to scientists. He describes briefly experiments which show that a white rat, a pumpkin, the new tail of a tadpole (when the first tail is cut off), a colony of yeast cells in a sugar solution, a colony of fruit flies in a milk bottle, grow in the same way as the populations of countries according to their census counts. That is, the rate of growth is slow at first, becomes rapidly faster and faster, and then after a time gradually becomes slower and slower.

Dr. Pearl goes on to produce evidence of different kinds on the factors which control population, showing: 1) that for some unknown reason, perhaps psychological, fertility decreases as the size of a group increases (for example a group of 50 hens in a pen will lay more eggs per hen than a group of 100 hens) ; 2) that, as if well known, wealth reduces the birth rate; 3) that poverty and hard conditions of life tend to increase reproductive activity. In this connection he produces statistics of the sex activity of some 250 married men at various ages. At all periods of life the average number of coitions per month was greater for the merchant and banker group than for the professional men, and greater for the farmer group than for either of the other two groups.