102 Life and Death, Heredity and Evolution 



when 1.1 parts of a standard solution of arsenic was added 

 to 100 parts of the water. By a gradual process they were 

 rendered resistant to 5 parts of this same solution to 100 

 of water. When they were again cultivated in fluid without 

 arsenic, they retained their resistance unchanged for seven 

 months, or at least 200 generations. But in the eighth 

 month it was found that the resistance was partly lost ; 

 they could now stand only 4 parts of the arsenical solution 

 in 100. The resistance continued to decrease gradually, 

 until at the end of ten and a half months they had entirely 

 lost the resistance to arsenic that they had acquired, so 

 that they were killed by the same weak doses that had 

 been destructive at the beginning. In many other cases 

 Jollos thus produced modifications of the power of resisting 

 chemicals, which thus lasted for months, but finally disap- 

 peared. He found that if the animals were kept under con- 

 stant conditions their resistance lasted much longer than 

 was the case if they were subjected to many changes of 

 temperature and food. 



One particular phenomenon did away immediately with 

 the acquired resistance; this was conjugation. Jollos found 

 that after the animals had acquired resistance to a consid- 

 erable concentration of arsenic, this resistance was com- 

 pletely lost as soon as they conjugated. To this fact Jollos 

 attributes a deep significance. He believes that it shows that 

 the modifications thus produced and for a long time passed 

 from parent to offspring by fission are in reality very dif- 

 ferent things from the permanently inherited characteristics 

 of the species. These characters the typical form, struc- 

 ture, and physiology are inherited not only in fission, but 

 also in the changes which follow upon conjugation. A 

 change in these characters, a permanent change in the 

 inheritance, Jollos would call a mutation, while these 



