THE CAUSES OF DEATH 117 



age period of 10-15, where it reaches its lowest point, and 

 only approximately 2~y 2 persons out of a thousand ex- 

 posed to risk die. The specific mortality curve then be- 

 gins to rise, and continues to do so at an approximately 

 constant and rapid rate for ten years that is to the 

 age period 20-25. From then on to the age period 50-55 

 it rises at a slower but constant rate. This is the period 

 of middle life, and here the female curve drops farther 

 below the male curve than at any other place in the span 

 of life. After the age period 50-55 with the on-coming of 

 old age, both male and female curves begin again to rise 

 more rapidly. They continue this rise, at a practically 

 constant rate of increase, to the end of life, which is here 

 taken as falling in the age period 95-100. In this last 

 class the rate has become very high. Out of 1,000 per- 

 sons living at the ages of 95 and 100, and therefore ex- 

 posed to risk of death within that period, 494 males and 

 473 females die, taking an average for the whole five- 

 year period. Of course, before the completion of the 

 period, practically all of the thousand will have passed 

 away. 



The important things to. note about this curve are 

 these: First, the highest specific forces of mortality oc- 

 cur at the extreme ends of life, and are higher at the 

 final end than at the beginning. In the second place, 

 there is a sharp and steady drop, in almost a straight 

 line, from the high specific force of mortality in infancy 

 to the low point at about the time of puberty. From 

 then on to the end of the span of life, the force of mortal- 

 ity becomes greater every year at a nearly constant rate 

 of increase, with only such slight deviations from this 

 constancy of rate as have already been pointed out. 



Turning next to the mortality of our first biological 



