VARIATION IN UNICELLULAR ORGANISMS 105 



them to be in essence of the same sort that occurs in growing 

 old, in the case of higher organisms. Others think of them 

 rather as damage to tiie constitution resulting from bad 

 living conditions. The essential facts for our purposes are, 

 first, that the changes are alterations in the hereditary con- 

 stitution; in the basic materials of the organism. And second, 

 that the environmental conditions play an important role in 

 bringing on these changes. In some species, indeed, if not in 

 all, a bad environment is the exclusive factor in bringing about 

 the degenerative changes; they do not occur if the animals 

 are kept in a favorable environment, but do occur if the 

 conditions are not quite those fitted to the organism. Such 

 decline of vitality, or depression, may be produced by sub- 

 jecting the animals for many generations to unfit food, to 

 scanty food, to too high a temperature, to the action of various 

 chemicals, and to various other unfavorable conditions. 



Thus the material of these depressed organisms has in the 

 course of many generations become gradually altered, de- 

 pressed, damaged. But what is of the greatest interest, this 

 damaged material continues to assimilate, continues to pro- 

 duce more of its own damaged material ; it grows and repro- 

 duces in the damaged condition, producing new generations 

 showing the depressed characters. For if we compare side by 

 side under the same conditions a stock that has been under 

 bad conditions with one that has not, it is found that both 

 reproduce, both multiply, but the difference between them 

 continues. Under the same conditions one multiplies rapidly, 

 at a high level of vigor, with high resistance, and good 

 digestion and assimilation; the other slowly, at a low level, 

 with poor resistance, poor digestion and assimilation. This 

 may continue for twenty, fifty, one hundred, five hundred 

 generations or more. While the materials of the normal indi- 

 viduals multiply themselves in the normal condition, the 



