72 THE EVOLUTION OF MAN 



is of advantage to the existence of the species. The third 

 theory, which was advocated by Maupas and Minot, holds 

 that, at fertilization, a stimulus is supplied to the developing 

 organism which is gradually dissipated, during the later growth 

 and differentiation, until finally none is left and the organism 

 dies of old age. As the phenomenon of old age is a common 

 one to all metazoan animals it would seem that the third 

 theory is more in accord with the facts. 



Biologists have estimated that an animal should live from 

 five to seven times the period of growth. This means that 

 man should live to be from one hundred to one hundred and 

 forty years old. The facts in regard to different animals, how- 

 ever, do not seem to support this view. Rubner has estimated 

 that man requires about four times as many food calories for 

 the period from the cessation of growth till the end of life as 

 other mammals and he interprets this as meaning that the hu- 

 man cells have a much greater total capacity for obtaining 

 energy from foodstuffs than those of other mammals. The 

 cells seem to be able to make only a limited number of chemical 

 transformations, after which physiological death ensues. Rub- 

 ner believes that the cells in man can make a greater number 

 of such transformations than those of most other mammals. 



VESTIGIAL STRUCTURES 



The normal life cycle of man having now been considered, 

 attention should be given to another valuable line of evidence 

 as to man's origin, namely, certain structural features known 

 as the vestigial organs which, although practically functionless 

 in man at the present time, are believed to be the remains of 

 well-developed, functional organs in the past. A few typical 

 examples may be noted. 



At the inner corner of the eye is a fold of the conjunctiva, 

 the mucous membrane covering the front of the eyeball, called 



