Heredity and the Origin of Variations. 175 



we are told, on the look out for every possible advantage, 

 would assuredly seize upon it and develop it. 



Those who believe in the absolute sway of natural 

 selection have not at present given any adequate answer 

 to the question How are particular variations (e.g. the 

 twisted skull of flat-fish) produced ? They say that con- 

 stitutional variations, which are alone inheritable, are due 

 to variations in the germs. When asked how these 

 variations are produced, they are forced to reply We 

 cannot say. But when it is suggested that they may be 

 in some unknown way transmitted to the germ from the 

 body, they are up in arms, and exclaim You have no 

 right to believe that, or ask us to believe it, unless you can 

 tell us plainly how the effect is produced. Unable them- 

 selves to give the modus operandi of the origin of particular 

 variations, they demand the exact modus operandi from 

 those who suggest that variations may arise through this 

 mode of influence of the body on the germ. 



We shall have to consider this question from a more 

 general standpoint in the next chapter on " Organic Evolu- 

 tion." We may now very briefly summarize some of the 

 results we have reached in this chapter. 



The ova and sperms are specially differentiated cells 

 which have, in the division of labour, retained and empha- 

 sized the function of developmental reproduction. 



There is a continuity of such cells. The cells which 

 become ova or sperms have never become differentiated into 

 anything else. 



Hereditary similarity is due to the fact that parents and 

 offspring are derived eventually from the same germinal cells. 



Variation in the existing world is partly due to sexual 

 union. But if there be mere admixture, new characters 

 cannot arise in this way, nor can old characters be 

 strengthened beyond the existing maximum. 



Some mode of organic combination (analogous to 

 chemical combination) might afford an explanation of the 

 occurrence of new variations and the increase of existing 

 characters. 



