198 THE THEORIES OF EVOLUTION 



"Paul Bert tried to acclimatise some Daphnse to salt 

 water by gradually adding salt to the aquarium. At 

 the end of forty-five days, when the water contained 

 1.5% of salt, all the adults had died, but the eggs in 

 their brood chambers survived, and the new generation 

 arising from these flourished well in the salt me- 

 dium." 4 Packard, a Lamarckian, who cites this case 

 from Cuenot, sees in it evidence of the heritability of 

 a modification, but Thomson, a Weismannian, regards 

 it merely as an instance of the direct modification of 

 the germ cells or of the embryos. 



The line is very hard to draw in the majority of 

 cases and the evidence demanded from the Lamarck- 

 ians is difficult to produce, for the two actions are not 

 easily dissociated in real life. And even if this could 

 be done, it would only have a theoretical bearing upon 

 Weismann's theory of the germ plasm ; it would have 

 no bearing at all upon the question of the heredity of 

 acquired characters as a factor in the evolution of 

 species. 



This distinction only assumes a certain importance 

 in the case of characters resulting from the use or 

 disuse of organs and generally localised. These char- 

 acters are considered as very important by the La- 

 marckians who have not been able thus far to account 

 for them. As far as all other cases are concerned, it 

 matters very little whether a modification is produced 



* J. A. Thomson. Heredity, p. 189. 



