252 DEATH AND THE DURATION OF LIFE 



butterfly, 1 year. Nevertheless, when this explanation is 

 critically examined many exceptions appear. A horse 

 lives 40 years, but so does a sea anemone or a toad ; a pig 

 lives 18 years, and so do some of the crabs. Mere bodily 

 size is therefore not the sole determining factor. 



Weismann's Theory. — Toward the close of the last 

 century, Weismann, a famous German zoologist, formu- 

 lated a theory that appears to be more in accord with 

 what is known of the subject. He emphasized the fact 

 that in many species of animals, all of the sex cells are 

 liberated at one time ; and, if the offspring are not subse- 

 quently cared for by the parents, the latter die at once 

 or exist for a short time only. Where the eggs are not 

 laid in one season, but during a period of weeks or years, 

 it is obvious that the span of life is more extended, but 

 here also it lasts but little longer than the time of sexual 

 maturity. Where the young are cared for the lives of 

 such parents are continued for a relatively much longer 

 time after the close of the reproductive period than in the 

 other cases cited. 



The number of individuals of a species remains approxi- 

 mately the same from century to century. There may be 

 fluctuations, one year being represented by many more 

 individuals than another, but in the long run the average 

 is practically uniform. This must mean therefore that for 

 every parent that dies there is another individual to take 

 its place, whether the reproductive period be long or 

 short. 



Application of the Theory. — The great length of 

 life of the elephant is thus correlated with the fact that it 

 is a slow breeder. It does not produce offspring until 

 twenty-five years of age, and at a maximum probably 

 does not give birth to over fifty. The eagle does not 

 build its nest until it is several years of age, and it has 

 been estimated that it does not lay over one hundred and 

 fi^ty eggs. In both of these instances the number of de- 

 scendants is small, but they receive parental care. On 



