96 THE FIRST PRINCIPLES OF HEREDITY 



the latter being variations which arise in the germ, though 

 their cause may not always be clear to us. Of such acquired 

 characters (acquirements they have been called in short) 

 there are many well-known instances, either due to environ- 

 mental effects, as the climatic changes of plants, the sun- 

 burnt skin of man, etc., or due to use and disuse, familiar 

 examples being the well-developed arm of the blacksmith 

 and the puny, stunted body of the factory worker. But in 

 practice it is not always possible to determine in individual 

 cases whether a given character is " somatogenic " (arising 

 in the body), or " blastogenic " (arising in the germ). In 

 fact, it is impossible for an adherent of Weismann to prove 

 that a given acquired character is not inherited, as a negative 

 cannot logically be proved in any case. All that the con- 

 troversy, then, turns upon is this : The instances adduced 

 in favour of the contention of the inheritance of acquired 

 characters have to be examined, each on its own merit. 

 As long as it can be shown that another explanation of these 

 cases is possible, they necessarily fail as evidence in support 

 of the contrary opinion. Indeed, the difficulty is, as 

 Herbert Spencer put it for his school, to find cases " where 

 the occurrence of selection, natural or artificial, can be 

 wholly excluded," 



We shall find it most convenient to deal with the several 

 points at issue by arranging them somewhat in the manner 

 Professor Thomson has done in his excellent book on 

 Heredity. 



(a) Inheritance in Unicellular Organisms. 



It has been argued, with an apparent show of truth, that 

 if acquired characters are not inheritable, then the whole 

 elaborate fabric of Organic Evolution falls to the ground, 

 as there would be no means by which the original one- 

 celled organisms could have changed and assumed new 

 forms leading to the more complex and higher stages of 

 life. Furthermore, experiments have been cited which 



