146 SUMMARY [OH. x 



have mentioned, two or more variations may be constantly 

 associated. In some cases the association is explained by the 

 fact that both variations are due to lowered vitality. For 

 example, the loss of power to produce pigment may be 

 associated with the loss of power to liquefy gelatin or to grow 

 on certain not very favourable media all these functions 

 being dependent upon the vitality of the organism. 



Again, the evolution or higher specialisation of an organism 

 may involve simultaneous modification in two or more direc- 

 tions. These modifications may all represent a casting off of 

 saprophytic characters by the organism in question on its 

 entry upon a parasitic career. For example, a saprophyte 

 may derive its vital energy from the sunlight by means of a 

 pigment, comparable to the chlorophyll of a vegetable cell, or 

 from carbohydrate food through its ability to ferment it. When 

 it becomes parasitic, and in many cases pathogenic, it is cut 

 off from sunlight and must subsist on the body fluids. We may 

 find therefore that the acquirement of virulence is associated 

 with the loss of power to form pigment and to ferment sugars. 



In the same way, the constant association between two 

 different variations may be due to the fact that the young 

 strains which show them have not developed their adult powers, 

 or to the fact that the variations are both signs of degeneracy 

 or atavism. 



"Impressed" variations show even greater differences in 

 their degree of permanence. In some cases a variation is only 

 maintained while the influence which caused it continues to 

 act. In others the variation persists for a shorter or longer 

 period after that influence is withdrawn. In others again the 

 variation is apparently permanent and persists under normal 

 conditions of growth indefinitely. We use the expression 

 "apparently permanent" for it is impossible in any case to 

 guarantee the permanence of the characters exhibited by a 

 strain of bacteria. This has been shown both by observation 

 and by experiment. Mention has been made elsewhere (vide 

 p. 14) of a strain of bacteria which after nine years' cultivation 

 lost its power to ferment maltose, and of another strain which 

 after five years cultivation lost its power to produce pigment. 



