Physiological Variation i6i 



numbers, such as stamens in roses, or ray florets in daisies, or 

 the number of segments in a worm or the number of veins 

 on a leaf, we shall find a similar distribution. This statement 

 of individual variation does not mean that no two individuals 

 have the same number of variable parts, but that within 

 certain limits an individual may have any given number. 



Physiological Variation 



No two cows will give precisely the same amount of 

 milk day by day, and of two quarts of milk obtained from 

 two different cows, the proportion of fat will be different. 

 Muscular strength varies among individuals. In a flock of 

 hens some will lay more eggs during the season than others. 

 The amount of sugar to be obtained from a pound of cane 

 or a pound of beet will vary from plant to plant. In Persia 

 it has been known for centuries that some horses can run 

 faster than others. In so far as we can measure these charac- 

 ters they also follow the curve of probability. 



Parallel to such variations among individuals are differ- 

 ences in immunity and susceptibility to various infections, 

 the ability to recover from injury or fatigue, in the capacity 

 to digest food in general or particular materials. There are 

 individual variations in the speed of response to stimulation, 

 in the ability to discriminate fine shades of color or fine shades 

 of pitch or of loudness. While we cannot measure such 

 differences as readily as we can the more obvious physical 

 qualities, they seem to be distributed in pretty much the 

 same way. 



The indications are that many emotional or tempera- 

 mental differences noted among human beings are also es- 

 sentially of the same kind. Some of them seem to be corre- 

 lated with variations in the amounts or proportions of 

 specialized substances produced in the body, such as the 

 insulin of the pancreas, the secretions of the thyroid gland, 

 of the adrenals, and so on. It is conceivable also that the 

 commonly accepted intellectual differences are due to com- 



