MENTAL AND PHYSICAL RESULTS. 269 



man has nearly doubled the length of time between generations of 

 our improved breeds of horses and cattle, and much of this im- 

 provement has undoubtedly arisen as a result of this increased 

 length of time. This increase of the time between generations has 

 not been deliberate, but has been the unconscious result of retain- 

 ing superior animals for breeding purposes as long as possible. 

 As these animals breed at all possible ages, and as selection is used 

 to weed out the undesirable individuals, it is only necessary to look 

 through the ancestry of the remaining ones to find the best average 

 age of reproduction. This age of reproduction is not to be deter- 

 mined by one generation, but by the average of several successive 

 generations. 



While man may not use selection upon himself in the same 

 manner in which he uses it on the lower animals, he may accom- 

 plish the same result by the use of his intelligence. The advance 

 of civilization tends to discourage early marriage, but the luxury 

 and idleness arising from the accumulation of wealth tend to 

 bring on sexual excesses which in turn lead to early puberty and 

 early marriage. Later reproduction, when unaccompanied by sex- 

 ual excesses, tends gradually to delay the time of puberty in 

 succeeding generations, and physical and mental activity in the 

 child not only tend toward the same result, but increase the de- 

 velopment of body and brain before puberty. The greater the 

 physical and mental strength before puberty the longer the indi- 

 vidual tends to live, and consequently the later is the age to which 

 reproduction may be extended. We thus have a cycle of causes 

 and effects, each of which may be intelligently controlled and each 

 of which tends towards a longer life and a higher and better 

 development. 



In the more advanced portion of the civilized races puberty is 



