REPRODUCTION, PUBERTY AND LONGEVITY. 235 



the head of a herd, a position which he maintains by means of his 

 superior strength. It is only with man in civilized and semi-civ- 

 ilized communities that the old become feeble long before dissolu- 

 tion, and this feebleness may be almost certainly attributed to his 

 own and his ancestors' vicious lives. 



As the degree of activity is normally sufficient for a continued 

 development to a late period in life, and as the degree of activity 

 for any given class of animals cannot be varied so greatly as can be 

 the period of reproduction, we may consider time as the principal 

 element in evolution. We may observe this among men, who vary 

 most in activity and by whom more children are conceived before 

 they arrive at twenty-four years of age than after they have passed 

 forty-four. No matter how extraordinary be the mental activity, a 

 man cannot acquire before the age of twenty-four a mental de- 

 velopment equal to what another man will acquire at the age of 

 forty-four by a very moderate degree of mental activity. 



EVOLUTION OF MAN AND THE HIGHER ANIMALS. 



Among the higher mammals, and especially with man, an in- 

 crease in time carries with it a gradual increase in mental activity, 

 so that whenever there is an increase in time before the act of re- 

 production, that fact will of itself cause an increase of mental 

 activity without the aid of any special mental stimulus. In other 

 words, time is itself a stimulus to mental activity, and whenever we 

 have an increase of time before reproduction, then we have a sure 

 progenitor of progress. 



Having proved that the length of time between generations is 

 the principal factor in evolution, we have an explanation of the 

 wonderful rise and fall of Greece and Rome, and an explanation 

 of whv some races have risen from barbarism and others have not. 



