114 THE THEORIES OF EVOLUTION 



ways assume the same crystalline type. Those differ- 

 ences in nature do not extend, however, to the anatom- 

 ical characters of the organs; from one end of the 

 body to the other, the physiological units of one given 

 tissue are identical. 



Thus heredity is easily explained: the reproductive 

 element, ovum or spermatozoon, is merely a little clus- 

 ter of physiological units endowed with the character- 

 istic polarity of the species. When those units are 

 found under conditions favourable to their develop- 

 ment, they group themselves naturally as their par- 

 ents did. This explains the transmission of specific 

 characters ; in cases of amphimixia, certain individual 

 idiosyncrasies result from differences between the 

 physiological units of the two parents ; there arises be- 

 tween those units a sort of conflict which blends in the 

 offspring the characters of the parents. 



It is more difficult to explain the transmission of 

 characters which are not innate but were acquired in 

 the course of life under the influence of external con- 

 ditions. How can such a modification of the adult so 

 affect its physiological units as to influence the sex- 

 ual product and thereby the offspring? Spencer ex- 

 plains that an organism is a combination of parts, 

 which, taken collectively, are in a state of unstable 

 equilibrium ; when the equilibrium is disturbed at any 

 one point, the disturbance spreads to the whole organ- 

 ism and the offspring of the organism thus modified 



