1 9 o THE HUMAN SPECIES 



as the result of their own attracting and repelling force, whose 

 intensity varies only with distance, the loss of potential energy 

 is always equivalent to the gain of kinetic energy (i.e., of work 

 done) and vice versa ". 



Moreover, the sum of the kinetic and potential energy 

 remains absolutely constant. " Plants," says Helmholtz, " con- 

 sume vital energy in so far as they store it up in the shape of a 

 particular kind of potential energy, namely, chemical tension : 

 animals convert the potential energy stored up in plants into 

 vital force." 



That man is no exception to this law is admitted, not 

 merely by natural scientists, but by every thinking man, for in 

 his whole living and being he is subject to the law of the con- 

 servation of matter and the conservation of energy. Man like 

 other organic bodies is fated to growth and decay, to a rise and 

 fall, and when the law that governs human growth is considered, 

 it shows a curve with a double wave caused by the appearance 

 of two separate periods of increased growth (E. v. Lange) : l 



(1) at birth as a continuation of the very active fcetal period; 



(2) at puberty. Lange has shown that to each of these active 

 periods succeeds a stage of quiescence : after the first comes a 

 slower but still progressive development; after puberty the 

 growth curve becomes changed to a horizontal. 



The law of human growth is clearly seen if we consider the 

 four generally accepted ages of man ; 



1. Childhood, with its merry, careless life spent in eating 

 and drinking, playing and sleeping, the stage being ended 

 among civilised folk by the going to school with its troubles 

 and restrictions. 



2. The ripening to sexual maturity, an age full of psychical 

 changes, marked, too, by enthusiasm for the "ideal ". 



3. The fertile years of married life, when the powers both 

 of mind and body are at their height. 



4. Lastly, old age, with a decline and increasing frailty of 

 body, and a blunting of the mental activity, the memory failing, 

 the train of thought no longer sustained. 



It would be quite incorrect to suppose that the span of 

 human life is materially different from that of animals ; they, 



i yahrb.f. Kinderheilknnde, Bd. 57, 1903. 



