94 Mutations and Evolution. 



CHAPTER X. 

 INHERITANCE OF ACQUIRED CHARACTKRS. 



In the last chapter it was pointed out that adaptational 

 recapitulatory characters have apparently not originated directly 

 through chromatin variations, but indirectly via the cytoplasm. 

 Under the influence of Weismann's conception of continuity of the 

 germplasm, the very possibility of acquired characters or impressed 

 modifications being inherited, was denied. Like so many other useful 

 biological conceptions this was pushed to an extreme, and a non- 

 existent degree of isolation and insulation of the germ cells from 

 the soma was freely assumed. But in the last decade there has 

 been an increasing tendency to adopt a more reasonable attitude to 

 these problems. Weismann's conception of blastogenic and 

 somatogenic variations or characters has also tended to lay too 

 great emphasis on a distinction which can scarcely be said to exist 

 at all in plants, except in sporogenesis, namely the segregation 

 between germ cells and somatic cells. The contrast we have 

 ventured to draw between karyogenic or nuclear characters and 

 organismal or recapitulatory characters, seems more in accord with 

 our present knowledge of the development, cytological structure 

 and genetic behaviour of organisms. 



In the meantime, experimental evidence for the inheritance of 

 acquired characters and related phenomena has been slowly 

 accumulating, but space will permit of reference to only a few 

 papers. We may first mention Agar's (1913) work on parallel 

 induction in the Daphnid, Simocephalus vetulus, where references to 

 the related literature will be found. Agar discovered that 

 when Simocephalus is fed on a culture of Protophyta the valves 

 of the carapace became reflexed, the degree of this abnormality 

 gradually increasing during successive instars or moults. If such 

 individuals were removed to normal conditions before their 

 (parthenogenetic) eggs were laid, these eggs nevertheless developed 

 into adults showing the same abnormality which their parents had 

 acquired ontogenetically through environmental impress. But the 

 effect soon wore off in later generations grown in normal conditions. 



Similarly, grown at higher temperature the animals were very 

 much smaller, developed more rapidly and produced smaller broods. 

 Eggs laid shortly after removal to ordinary temperature developed 

 into adults nearly as small as their parents, but in F 2 little of the 

 effect remained, 



